Rachel's techniques for sewable circuitry: seems somehow soldering SMB parts to make them sewable. Related idea is flexible printed circuit boards.
Use MOSFETS instead of bipolar diodes: the voltage drop across a standard diode is about 0.7 Volts. It seems it's possible to use a MOSFET (metal oxide over semiconductor field effect transistor) as a diode in drain follower mode. The voltage drop is much less than across a standard diode. as more amperage goes through the diode, more power is wasted. also for circuits with very low voltage power sources, because there is very little voltage drop, more voltage is available for the circuit load. I'd like to build a circuit or two to test this. Applications include remote, low-power systems. http://electronicdesign.com/Articles/ArticleID/19871/19871.html
"Ultimate Continuity Tester", per Electronic Design, October 2, 2008: The article claims normal continuity testers trigger through connections of as much as a few hundred Ohms, which sometimes is too crude. Sometimes what's needed is a continuity detector that will not trigger through as little as ten Ohms. The circuit described is cheap, robust, and very sensitive. I'd like to try to build this. http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=19768&bypass=1
Amplifier for function generator: it's nice to have an amplifier and speaker combo around for testing. There are various ICs that are pretty good audio amps, given a power supply and a few components.
Audio mixer: it'd be a fun hack to mix the outputs of function generators and other audio range gizmos.
Audio cube: the idea is eight independent audio amplifier-speaker combos in each corner of a cube with a mixer that could send signals to any combination. the control for the mixer seems tricky: a multiple pot comes to mind as does a cubic container with semi-conductive liquid with terminals in each corner of the cube and a stick with an end that presents a signal source to the liquid. There's always the CPU method of spitting bit slices to each corner amp per user-interactive controlled algorithms (the GUI or CLI user interface seems problematic).
Power test resistors: a few 100 Watt resistors of various sizes could be useful.
Multiple voltage power supply: a single device (i.e. a single 120VAC power plug) that provides various positive and negative voltages, probably most limited to less than 1 Amp.