Quake Live costs money now and has less features than it did prior:
1. Stats are now relegated to a third-party website ( qlstats.net ). However, I have found qlstats to work well.
2. Matchmaking is no longer included. Most people seem to think it was bad anyway, but it worked well for me. You have to manually look at people's Elo with extraQL in the Workshop, and compare them to your own. An annoyance with extraQL is Steam often tracks Steamworks SDK Redist instead of Quake Live.
3. Only runs natively in Windows. Runs perfectly in wine though.

ioquake3 is free/libre which is nice if you care about that, but unfortunately finding real people there is an exercise in frustration. It's servers are riddled with bots and they don't show up as empty in-game. Meanwhile, Quake Live has the largest player base of any arena FPS, let alone Quake game. This makes Quake Live the way to play Q3A online nowadays.

UPDATE 14.4.2016
There is now fan-made matchmaking: https://t.co/oiSFs1TxfR

UPDATE 3.1.2017
Version 2.26 of extraQL no longer shows up as "playing Steamworks SDK Redist" in the friends list or under "Library / Recent"

Quake Live costs money now and has less features than it did prior: 1. Stats are now relegated to a third-party website ( qlstats.net ). However, I have found qlstats to work well. 2. Matchmaking is no longer included. Most people seem to think it was bad anyway, but it worked well for me. You have to manually look at people's Elo with extraQL in the Workshop, and compare them to your own. An annoyance with extraQL is Steam often tracks Steamworks SDK Redist instead of Quake Live. 3. Only runs natively in Windows. Runs perfectly in wine though. ioquake3 is free/libre which is nice if you care about that, but unfortunately finding real people there is an exercise in frustration. It's servers are riddled with bots and they don't show up as empty in-game. Meanwhile, Quake Live has the largest player base of any arena FPS, let alone Quake game. This makes Quake Live the way to play Q3A online nowadays. UPDATE 14.4.2016 There is now fan-made matchmaking: https://t.co/oiSFs1TxfR UPDATE 3.1.2017 Version 2.26 of extraQL no longer shows up as "playing Steamworks SDK Redist" in the friends list or under "Library / Recent"