Korg Volca Mini-Synthesizers
A few years ago I saw the Volca FM mini-synth advertised and the price for what it did seemed pretty good, so I purchased one. Can't remember the exact cost but I think it was somewhere around $100. There are a number of Volca models around now but I've only purchased the FM synth so far, and still have quite a ways to go to master the thing. There are a good number of videos online that demo them or instruct on how to use them.
Although there are a lot of modders doing all kinds of things to these I want to keep mine stock, at least for now. These have MIDI interfaces so are quite versatile in their use, pretty much like a larger synth. I've used mine with one of my synth keyboards, just controlling it via MIDI from the keyboard and it's quite capable for a $100 synth!
I will not have much more on this page, other than what's here, for a while unless I find some good deals on other Volca models.
Here's the face (top) of the Volca FM - the controls look decievingly simple but there is a lot of capability especially in that 'Program' control and the auxilliary controls you can see on the keyboard. A lot of this I still have not played with... I just need to spend some quality time with it and try everything out eventually, which is actually a pretty tall order.

There is a battery compartment on the back for self contained power or it can be powered by an external 9VDC power adapter.
This is one of the programming reference cards that come with it - you can see quite a few parameters that can be manipulated here! There is also a manual of sorts that comes with it.