Here are some hobby projects I've done involving measurements and electronics.

Time  measuring elapsed time, time intervals, frequency, and so forth

Zipit Z2  small, cheap, has wifi, runs Linux

Light Levels using Arduino-compatible JeeNode with LuxPlug (Nov. 2010)

Battery Charging and discharging curves (NiMH)

USB power from external battery pack (for HiMD device)

Audio measurements of digital recorders.

Fuji F700 vs Canon D60 image comparison.

Canon D60 digital SLR intensity transfer curve.

Vivitar 285HV 12V supply for a faster flash recycle time: pro and con.

Sharp MD-MS702 MiniDisc recorder noise.

Aiwa AM-C80 MiniDisc recorder noise.

Geiger Counter radiation measurement.

Geiger Counters for measuring alpha, beta, gamma & X rays.

GPS measurements

Car Counter measuring car speed/size/acceleration.

Fluxgate measuring magnetic fields.

Spectrum of a compact fluorescent light.

Spectrum of a UV LED flashlight.

Floor Fan output measurements.

Akai DPS16 modification for lower acoustic noise.

Ballistics project long ago, a few pages from it.

Source miscellaneous hardware-interfacing source code.

epanorama.net is a very useful reference on many aspects of electronics.