Rigol DG1022 3.7" LCD 20MHz Function/Arbitrary Waveform Generator

Pros: Small, versatile. Generates waveforms from scratch or manipulates incoming ones. Graphing functions visualize the waveform being output on the built-in dot matrix display. Combined with an oscilloscope of the same brand the device becomes a signal duplicator/re-shaper.

Cons: Important specs omitted by DX:
Max output voltage is 20V. For anything above that you'll require a separate signal booster (which DX doesn't sell yet as of 2011).
The maximum square wave output frequency is 5MHz (1/4 of the 20MHz bandwidth), so if you plan on replicating, say, quartz crystal response time, keep that in mind. 5MHz is as high as you'll go.

Other: What can you do with 20MHz? Basically it's enough to generate audible tones. Enough to control servos and emulate stepper motor drivers. Enough bandwidth for low fidelity video sync emulation. The square wave falls 1MHz short of standard definition NTSC bandwidth (perhaps on purpose), but unless you're specifically into video stream tinkering, it shouldn't bother you. Spectrum gear costs thousands of dollars.
Excellent item that may actually save you money through its ability to emulate some electronic components.
Ideal for testing error tolerance in prototypes (injecting noise into the current). Good item for a microcontroller hobbyist to have.