Doepfer A-151

If you are yet to get your head around what a sequential switch does and how you'd use it, the a151 can take an input and route it to up to four different outputs (or vice versa, can be used the opposite way round to route four inputs to one output), selected sequentially by sending a trig. There is also a reset trig input for jumping back to the first step (useful for resetting polyrhythms, for example).
"So what?", you may be thinking...

The awesome power of switches becomes apparent when you get your hands on one and start patching. Some ideas:
Put 4 different audio rate oscillator waveshapes to the four inputs and advance the steps at audio rate, you now have a nice complex wave coming out.
Put 4 constant cvs to the inputs and you have an up to 4 step cv sequencer, great for arpeggios when the ouput is mixed with a v/oct signal and sent to an oscillator.
Send multiple clocks to the 4 inputs, stepping through rhythmically, and send the output to some logic - incredibly complex drum patterns abound.
Send clock to input one and mult it to the step advance, leave inputs 2-4 empty - now you have a clock divider, division selected by the number of inputs switch!
Take your single synth input and send to four different fx configurations, step through them for each section of your song structure (verse/chorus/bridge etc) - switches are amazing for creating complex song structures that you just couldn't do without them.

And this all just with one 4 step switch - add another and the possibilities increase exponentially. Add its bigger brother, the a152 (which is 8 channel and addressed, so can pick the steps sequentially but also ad hoc as needed via cv) and its just insane how much you can do with it. Switches are a must have for complex patching, and you cant go wrong with doepfer for the price vs power vs quality.

It took me a while after starting my modular system to realise how useful voltage controlled switches could be. The manual offers quite a few suggestions.

I mostly use this for transposing sequences using a divided down clock signal to shift a sequence making an eight note sequence into 32. I also have set it up for sequencer ratcheting using various clock signals to add interest and rhythm to a simple sequence. Being able to choose whether you use 2, 3 or 4 steps also adds complexity. I often use this switch as a performance control. Audio signals come through clean and clear. It'll happily run at audio frequencies to make interesting wave forms and it has the all important LEDs to let you know what it's doing.

This module is neat small and inexpensive - lots squeezed into 4HP.

This is an example of a module that does a specific thing very well.

You can use it to route four things to one destination, so you can have different vibrato rates for each step of a sequence, or you could route one thing to four destinations - take an LFO and modulate the cutoff frequency of a filter on step one, the resonance on step two, the amplitude on step three and the pitch on step four.

The output (or input) is freely selectable for two, three or four steps so interesting rhythmic variations can be achieved; the reset input returns to step one, so you can restart the cycle at the beginning of the bar; or indeed, anywhere you choose.

This switch is at the bottom of the unit and is freely accessible.

The build quality is great, it arrived well packaged and had two screws to mount it into the case.

No manual, but this is available online. If you want to send signals around your modular synth in a reasonably inexpensive way then this is a great starting point. It may not be as fully featured as other sequential switches but it doesn't take up too much space and is rock solid.