I'm surprised no one mentioned Eligius.
It's by far the simplest pool : no registration (and thus, no security issues, hello Deepbit !), you directly get the generated coins (no intermediate transaction). And, as an added bonus, no fee !
It also has nice individual statistics
, and a very active IRC channel (#eligius on Freenode) for discussion and support.
I use btcguild via PNLNS. It runs about 103% luck over past 6 weeks, so the 3% fee is null if you mine that long. Nice UI. Merged mining with NMC. So does Eligius. Eligius is my backup. I like how the new coins are original. Very cool to see them coming in on blockchain.info. It's like boinking a virgin when you spend them, haha!