Lately I've picked up where I left off plugging into random amps by wiring inputs directly into toy radios, walkmans and boomboxes. Every different radio has it's own distinct tone, and some of my home-wired devices sound better than anything I've ever played through. Here's one of my first prototypes, a toy sing along cassette player. I simply removed the mic and replaced it with a 1/4 inch input. This doesn't have much in the way of volume, but sounds great with any effects pedals I've tried.
Cool old boomboxes have become easy to come by lately, as the tape motors eventually fry, or the belts rot. A cassette boombox makes a great amp. There are a few ways to wire these up, but for this one, I rerouted the three wires that go to the tape head, and ran them to a jack. Hit the play button to turn the amp on. This thing sounds so good, I couldn't believe it when I first tryed it out. The guitar sends a stronger signal than the tape did, therefore distorting the signal, giving it a great overdrive effect.
Here is what this sounds like, no effects whatsoever were used. Gibson SG to Boombox amp to cassette:
Another radio made into an amp. This time I figured out how to actually wire into the radio instead of going through the tape head or mic line. Plugged into it is a great little Fisher Price phone toy I got at a junk shop. The only mods I did were adding a line out after I took of the face of the toy. This is easily the coolest toy I've found yet. When you put a battery in it, it just starts making random bleeps and sqwaks, and all you have to do is touch the circuits to manipulate it. It's even sensitive to movement, whether you move the toy itself or just put your hand near it without even touching it. Like I said earlier, the older and more archaic, the easier it will be to bend.