I play FreQuency, Amplitude, and Final Fantasy XI: Online.
FreQuency isn't exactly "popular," as you'll only find around 20-30 people online at a given time, and most just sit in chats (the bastards). It's also not online out-of-the-box (you have to do a disc swap between the Network Adaptor disc & the full game), kind of old, and one-upped by Amplitude... So it's kind of hurt by all that. Fortunately, the NA start-up disc has an online-enabled demo on it, so you can really see the status of the online game, and play about 4 (IIRC) songs online; it's worth checking out.
Amplitude is definitely more popular than FreQ. Typically you'll have a full game every few minutes (rarely happens in FreQ anymore
), and there's a lot of good competition. And unlike FreQ, Amplitude was built with online play in mind, so there's a lot more game options available, a ranking system, and no disc-swapping. There's also a download center where you can download mixes Harmonix and other players have made, so it ups the replay value and keeps things fresh online.
Lastly, Final Fantasy XI: Online is uber-popular as I'm sure everyone knows. As of April it had over 500,000 paying subscribers, 1.2 million characters created, and so on. So you won't ever have problem with not enough people playing. But there's also the extreme costs in mind (HDD, subscription fees, etc), as well as the whole hit-or-miss MMORPG thing. It's a lot to consider, so it's totally up to you if you'd find it worth it.
That's all I've got for now. For upcoming I'm really looking forward to Gran Turismo IV, Monster Hunter, Ambrosia Odyssey, Front Mission Online, Champions: Return to Arms, and of course, Final Fantasy XI: Online: Chains of Promathia (expansion pack). It may not seem like many games to some people, but it's more than enough for me.
