For PCs, it's a no-brainer: NewEgg. Actually I used them for all my computer parts, including power supply unit, DVD burner, fans, memory, graphics cards, hard drives and so on. I have yet to have a problem with them and they are always the first place I check for good stuff at low prices (in retrospect, I was just looking at the list of purchases I made from them in the past year and realized I spent a lot just there).
I'm pretty new to the world of Mac. While you just have to match up the memory speed for a PC, I didn't know if the memory at NewEgg would be Mac compatible. I bought a 1GB stick of memory for my iBook, snapped it in, and had absolutely no problems. And for guaranteed compatibility, it wasn't much more expensive than at NewEgg.