Many beginners say they're interested in making "drones" with their modular synths - musical textures that eschew conventional musical notions like rhythm and melody. I think part of the appeal is the (not entirely correct) perception that drones are easy and don't require musical ability. It's also a fact of life that if you have very few modules it's a lot easier to make drones than anything else. Drones are fine, and I make a module especially adapted to them, but if you want to take the next step and make music with notes in it, you need something to trigger and control the notes, and that often means a sequencer.
Eurorack modulars are usually controlled by two voltages: one for pitch (on the V/octave plan) and one "gate" voltage which goes high (typically 5V) during the note and low (0V) for the silences in between. Some other modular systems also use separate "triggers"; the note is started by a short pulse on the trigger voltage and then remains playing as long as the gate voltage is high. In Eurorack there is no clear distinction between a trigger and a gate, but we still refer to voltages as triggers if they are meant to activate things like drum sounds that control their own duration.
To play notes on the synth, then, you need something to generate the pitch and gate voltages, and that comes down to one of two things. You can use a controller to manually create the notes, whether it is a keyboard, ribbon controller, theremin antenna, or something else; or you can use some method of generating notes automatically, usually according to some pre-determined plan. That is the realm of the sequencer.
Classic step sequencers
The most traditional kind of sequencer for modular synthesizers looks something like this Doepfer A-155 (US$375).
This is what's usually called a step sequencer (though that can also refer to more complicated types), or sometimes (but incorrectly) an "analog" sequencer. Note the row of LEDs along the top. The module's operation is very simple: it has eight steps and on every beat, it goes to the next step, so it's repeating a cycle of eight steps. There are two rows each of knobs and switches, allowing you to set a couple of control voltages and some digital outputs for each step. You can set up a repeating eight-note melody with these, or (because there's also a row of control inputs that influence the output voltages for individual steps) do something a little more complicated with some of the steps changing over time.
The good thing about this kind of sequencer is its versatility. It's a very modular way to do sequencing - breaking the task down into the simplest possible operations and then allowing you to build up more complexity by adding modules. The down-side is that it has a very strong tendency to push the music to consist solely of repeating loops. If you're making EDM, fine; if you're making something else, it may not be the emphasis you want.
Arguably even simpler are trigger sequencers, which despite the name often do gates as well, but not variable control voltages suitable for pitches. These are handy for drum rhythms. They're often packaged in small modules that don't take up much space, and are controlled with a push-button menu interface. A typical example is the Intellijel µStep (US$195), which fits in just 4HP.
More complicated sequencers
Many popular modules offer different computer-controlled wrinkles on basic sequencing. One I like is the Tiptop Z8000 (US$450. Mark I panel below; the current version has a less photogenic panel). There's a grid of 16 knobs but instead of just reading them out in sequence as a 16-step loop, there are a bunch of independent virtual "sequencers" that read different combinations of the knobs independently (rows, columns, and two different reading orders for the whole grid). The result is multiple separate but related patterns from a single set of controls.
The Make Noise René (US$500) is a combination of a sequencer and performance controller, with a built-in touch keyboard that allows "playing" it as well as using the automatic sequencing. The touch keyboard (based on skin conductivity, not capacitive sensing) is something of a bugaboo for this module - it doesn't always respond well, and some people find they need to moisten their fingertips to get it to respond at all. But the module remains one of the most popular sequencers on ModularGrid.
Moving more in the direction of planned composition, Orthogonal Devices out of Japan offers the ER-101 sequencer and the ER-102 expander (US$600 and $450 respectively), which offer extensive control and editing of stored sequences in memory, through a unique interface based on rotary encoders and multiple seven-segment displays. This pair of modules is somewhat polarizing - people either like it or hate it - but it's probably the most sophisticated in-rack sequencing available without bringing in a full-blown general-purpose computer.
Random and automatic sequencing
It's a standard patch, something of a cliche, to run a noise source into a sample and hold module to generate a stream of random notes. The result is something like a step sequencer output with no fixed pattern to the melody. If you want a little more structure to your music but still something basically random at heart, there's the Turing Machine (about US$120 for a do-it-yourself kit). It's essentially a trigger sequencer that randomly changes its pattern over time. This one is not normally available as a finished module, though you may be able to find someone to build one for you if you don't want to build it yourself. "Expanders" for the Turing Machine allow it to also generate pitch control voltages and support other added features.
The Klee sequencer (pronounced like "clay," for the painter) is another take on automatic sequence generation, something like a cross between the Turing Machine and a basic modular step sequencer. It looks a lot like an A-155 with rows of LEDs and knobs, but instead going through the steps one at a time, the Klee can turn on several steps at once, and the whole pattern shifts and changes with the clock. The voltages for the active steps are added up to generate the output. This is another one primarily available as a do-it-yourself project, and I'm not sure whether anyone is currently offering kits, so it may require a bit more effort and research to build your own.
Sometimes people just want to plug in a module and get useful sequences without a lot of programming, and there are modules designed for that. The term "Euclidean sequencing" in modular synthesis refers to a way of generating beat patterns that is not closely connected to the work of Euclid at all, but never mind, we're stuck with it. Enough of the patterns generated sound musical that there's value in a module specifically to generate them. The Rebel Technology Stoicheia (US$225) is a good example - you can just plug in a clock, set the knobs to the sequence length and density you want, and get a reasonable drum pattern. The Mutable Instruments Grids (US$230) is a somewhat more complicated module in a similar vein: it's like a wavetable for rhythm, with a bank of preset drum patterns and control voltage inputs to move between them.
Sequencing via computer
Although many wigglers try to avoid connecting a separate computer to the synth, it's really the most efficient way to play preplanned compositions. With a simple MIDI interface module like the Doepfer A-190-3 (US$140) or a more complicated one like the Mutable Instruments Yarns (US$360), you can put your modular synth under control of a software sequencer like those I listed in my rundown last November. If your taste runs more in the direction of DAW software like Ableton, Expert Sleepers has a whole series of modules with associated software plugins for interfacing between the DAW and the rack.
Something I'd like to see more of in the modular world is the use of custom-written software to do automatic sequence generation through MIDI. For example, here's one of my semi-automated compositions generated by homemade software sending notes to the modular as I adjust the knobs and do some repatching.
There's a lot further that sort of thing could be taken.