||UT2D for UT3 is a mod that turns it into a 2D shooter. Basically, instead of looking out the player's eyes, you're looking at the side of the player while he runs left or right. Think like Mario, but UT3. Aside from all that, this is basically UT3. Most of the gametypes and weapons are all there. UT2D lets you go head-to-head with the game's AI bots or take on two-dimensional human opponents in Deathmatch, Team Deathmatch, King of the Hill, and Duel modes. No need to sweat things like ambushes or snipers because it's impossible to hide in a 2D side-scroller. You can move left or right, jump, and duck using the standard WASD keys while aiming and shooting with the mouse. There are plenty of power-ups (weapons, ammo, health, and bonuses) littering your pathways but you'll need to hone your jumping skills to access some of the better ones as they're often tucked away on high platforms.
UT2D Super Mario was a map created by a modder named FrontAXL that attempts to capture the look and feel of the famed '80s NES game Super Mario Bros. It does quite a good job at this too because the blocky platforms, bouncing coins, and even the chintzy music immediately (and quite effectively) trigger memories of the 8-bit arcade classic. You don't get to play as Nintendo's famous plumber (this is just a map, not a total-conversion mod) but it's still a lot of fun maneuvering your body armor-clad space marine through all the moving platforms, big pipes, and momentum-slowing brick walls.
In the Killing Time maps, a big master robot sends evil lesser robots through time and space to destroy the human race. You must travel through time to kill the robots before they can accomplish their evil goal.