Another commonly-recommended place is DMS:
http://www.datamemorysystems.com/_memory-information/4GB_Kit_DDR2667_SODIMMs_190x2.asp
...whose prices are usually nearly identical to OWC, and they also offer guaranteed-to-work-in-a-specific-Mac RAM and a lifetime warranty. Been using DMS for over a decade at work, and the two times I had memory fail they were quick and easy with a replacement. (Interesting aside: When you call DMS support, somebody picks up the phone. As in, no phone tree or automated system at all--that's always a pleasant surprise.)
That said, I'd get the OWC stuff. I switched to them at home a couple years back, for the combination of their very good
green business practices combined with decent prices, good quality, and lifetime warranty (which means something because they've been around for a long time).
Not saying G.Skill is bad or anything, just that I'm willing to spend ~10-20% extra for peace of mind and a company I like very much.