When it's RAM for my personal machines?
I ONLY go to Crucial, which is the retail arm of Micron. They OEM for lots of other brands of RAM.
All their ram has lifetime warranties, they guaranty the ram you buy will be compatible with your mainboard (you can select almost any brand and model of PC or motherboard from their site, and they show what's compatible), their prices are usually about as good as Newegg, and shipping costs next to nothing and always Fedex 2-day or better.
I know this sounds like a commercial. Sorry.
I buy tons of stuff from Newegg and Fry's, and I have no complaints with RAM from those guys, but Crucial really is the best way to go for RAM.
http://www.crucial.com-Llama