SRS SR850 lock in amplifier. It was thrown away because half of the CRT screen was black and unreadable. Someone even wrote on the case "GPIB ONLY IN PRINCIPLE". I had to bend some metalwork, but this I suspect was the damage caused by the workers chucking this boat anchor into the dumpster. The real problem (CRT image) was just soot and fine dust collecting on the acrylic in front of the CRT tube, as the high voltage in proximity likes to attract such dirt. Some cleaning and SR850 is good as new, well within spec (under 6 nV/sqrt(Hz) input noise).
Also few E3631A. One had faulty 30K ADC resistors (mentioned by free_electron somewhere here), another one just grease in the rotary encoder and the third one took the longest to debug - faulty SRAM causing the CPU to crash.
That's pretty much it from the dumpster. On eBay I once picked TTI TG1010A for 35 GBP. It needed a replacement battery (non critical, just waveform storage & settings) and had a cold solder joint on one of the SOT-23 transistors (AFAIR) used in row/column matrix switching for the keypad (the first column of buttons didn't work).
Surprising amount of eBay "for parts or non working" purchases turn out to be perfectly fine. I'm not TheSignalPath, I cannot afford to risk $500 on something broken (well, I cannot afford $500 on anything most of the time anyway), so these are rather really bottom of the range scores, $30-$50 most of the time with few exceptions in the $100-$150 range where I had some gut feeling (or judging from the photos) that the thing is OK or fixable.