I play there. My name is (hide your surprise) BLReid. It is a great site, but I probably won't renew my membership when it expires. It came with Fritz8, which I also love. My chess budget isn't that big though (my wife just doesn't see it like I do), so I have to limit myself to 1 pay site (here!). 🙂
I also use playchess, and have paid for a year (having got a trial with chessbase). Name is the same as here. I don't use some of the features, I've just played in a couple of tournaments. Very good site.
I used to use ICC (www.chessclub.com), which also has a lot of GMs etc (including, so the rumour goes, Kasparov, for a while), but there are just too many nutters on that site now.
chess 21, which is excellent, is not free. I've got Fritz 8 which gives me free access to playchess (forever i guess). I agree with Redmike; there are vast numbers of marauding nuts on ICC. You just have to learn to avoid them, as in real life. And if you run into one, don't play their juvenile games.
ICC (internet chess club) is by far the most interactive site around. Playchess has too much lag for me, as im in thestates, and it seems the servers are in germany? Ive been playing on icc for 5-6 years, and i like it. There are a lot of admins to help you, and a great interface (blitzen) if your ready for the $49 yr fee (1/2 for students) then play there. Im grandmouster there too, so if you join, mention me as the member who refered you 🙂