An open source engine from 2004 focused on an elegant engine design that allows for simple realtime map editing with no pre-complilation, while making better use of modern graphics cards than its 2.5D predecessor Cube.
Has a small but still-thriving community and many user-generated maps.
Pro Many gametypes and gametype combinations
Free-For-All (everyone for themselves, all weapons allowed), Capture (where teams fight for control of points on the map), Capture the Flag (two teams fight to capture the other's flag and return it to their base), Teamplay (defeat the other team's players to score points for your team), Tactics (FFA, no weapon pickups, players spawn with random equipment), Efficiency (FFA, no weapon pickups, players spawn with all equipment) InstaHold, where two teams have to possess a single flag for a minimum of 20 seconds to score points; Collect (kill enemy players and collect their skulls, which then have to be returned to the home base), and Protect (teams try to touch each other's flag). Instagib, regenerative weapons ("regen") and Teamplay versions of most of the game modes are available, as well as online cooperative map editing—one of Cube 2's most interesting and popular features.
Pro Easy map editing
Getting into map editing is very easy, which has resulted in a large number of user-created maps with a wide variety, from massive sky castles where you have to hunt down your opponent from far away, to large CTF maps, to competitive, claustrophobic 1v1 arenas with teleports.
Pro Simple gameplay
There are no skills beyond those common to all AFPS games: very basic movement, aim, weapon switching, item timing, map knowledge. There is less to learn so newbies can become proficient more quickly, but there is still the ability to hone those basic skills to a very high level, as evidenced by regular community tournaments.
Con Simple gameplay
The gameplay is more basic than almost all other AFPS games. Movement doesn't get any more advanced than recoil/rocket jumping, without even crouching or bunnyhopping. This is great for newbies and for becoming competent quickly but for experienced AFPS players getting around the map is less interesting and there are less skills to work on - just the basics of aiming, weapon selection, item timing, and map knowledge.