Online games, you say? I've played just about every rhythm based online game at some point in my life (SMO, DJMax, O2Jam, etc), as well as just about any MMORPG you can name. My MMORPG addiction does not stop with English games either, as I've played just about every Korean MMO that's come out over the past 5-6 years. There's far too many to name, but all the MMO's mentioned in this thread I have played at one point or another, regardless of if it was for only a couple weeks or a couple months.
MMO's that I've played extensively are few and far between, so I'll mention ones that I've actually gone through the trouble of paying for, whether monthly or cash shop. They are (in no particular order): Helbreath, Trickster, Albatross 18, Hellgate: London, Ragnarok, World of Warcraft, Phanstay Star Online, Phantasy Star Blue Burst, Phantasy Star Universe, Lineage II, Final Fantasy XI...that's all I can remember. Among these, World of Warcraft has held my interest the longest. I played it for 5 months when it came out, quit, came back about 6 months later and ended up playing it for ~1 1/2 years. I quit again after that point, but I got back into it briefly a couple months ago when I rerolled onto my brother's server...though I've pretty much stopped playing it again. Either way, no MMO has managed to keep my interest for longer than 3 months aside from WoW.
I've played all the other mainstream MMO that I didn't actually pay for monthly like EQ, LotRO, etc, though...mostly during beta periods. Currently trying to get into the Conan and Warhammer betas, although I hear they're incredibly boring at the moment.
Recently I just bought myself the Orange Box (finally) and have been playing the hell out of TF2, as well as Unreal Tournament 3. I've never been a fan for FPS's, but TF2 has managed to hook me quite hard. The Unreal series I play often, also, as I've been playing that on and off since it first came out. UT3 is definately the best Unreal released since the original.
Oh, and as far as WoW private server popularity...I've never actually played on one, but they are starting to run as rampent as RO and ROSE servers have in the past...meaning they're everywhere. Just need to know how to look for them.