What those guys said. Here's some links:
Admiral's Command Chambers, a big site devoted to SP user made levels (many reviewed).
Masssassi.net's levels section (with some mods too), biggest collection I've seen thus far (of course they don't have everything, but a good selection).
Our very own HyperView, lots of stuff, not updated much lately (long story), and not all organized, but plenty of stuff and reviews too.
Levels and reviews, again not updated very often recently, but plenty of levels and reviews (check the "library" section after choosing your "side").
JKMAG's BFP2X page, get a couple of great big skinpacks here. JKMAG also has some links to other patches and such on the site. There is a link to patch commander here, but if not, go here
Rbots website, with bots for JK and MotS (in development, but you can download a beta version and send the author feedback!). A new version is coming "soon" whenever the site issues with Massassi and their new host get resolved.
There are I'm sure other level pages out there, but those are the biggest that I have seen that are still navigable. ; )
Lots of folks still play on the Zone, a few play on GameSpyArcade (you have to track them down and invite them into the game mostly) and people continue to use the IP boards/Qtracker at JK.net ( http://www.qtracker.com
Qtracker is a free program that can scan the IP boards to see if games are up, and also to host your own games. You can have it automatically update the IP boards with your game too when you launch directly from QT).
The server boards can be found here:
If you play on the Zone, chances are you'll run into cheaters, it's a fact. So get Kicker Helper and know how to use it!
I'd recommend saving this html page (ie: in IE go to "file" then "save as" and save this thread to your hard drive, then you can refer to it later, even offline).
Good luck! ; )