In the past I've sold off most of my collection in waves - to free up some space, make a bit of quick cash and because I just wasn't playing the games regularly enough. Same as most people I guess.
Home computers went first (Spectrum and Atari ST), followed by NES, Master System, SNES, Megadrive (and Mega CD), Saturn, PS1, Dreamcast, N64, PS2, XBOX and 360. The Dreamcast hurt a bit. The SNES hurt a lot. Got rid of my arcade cab and jamma boards too (still miss R-Type and R-Type II even though I can play them on any number of systems now).
Handhelds are where I have the most regrets. I had an almost complete UK Neo Geo Pocket collection that I sold off - I really wish I hadn't done that (and not because of the money, although some of the prices are eye watering now). Ditto my Gameboy, Gameboy Colour and Gameboy Advance games - still wish I had them now.
I guess it's cyclical because now I'm eyeing up the Wii/Wii U and PS3 as next on the list for a clearout. PS4, PSP, Vita and 3DS are still in use and Gamecube I'm keeping for now as that would still fall firmly into the 'regret' category.