I am not sure how I missed these two great mods, but they're really fun :D It's very Doom-like in that you have to kill tons of enemies but the levels in the first one are very basic. I recommend you play 'Gateways' before The Great Forever Tommorrow.
“This is a single player Half Life 2 mod, set entirely outside of City 17. It features6 levels of action, exploring a range of locations (from an underground facility to fighting alongside the Combine on an alien world to floating color-coordinated cubes), non-linear level progression (you start near a nexus of gateways, and may choose the order in which to progress through the levels), a musical score (a total of 8 tracks), some new abstract textures and challenging gameplay. This mod is meant for those who have already completed Half Life 2″
http://www.planetphillip.com/posts/gateways-half-life-2/
What I find most interesting is the "Nexus" map. It consists of a center with power-ups and health, with portals that lead to other themed maps. When you complete a theme level you are warped back to the center of the nexus until all levels are beaten which unlocks the final level. The music is really catchy. One of my all time favorites.
After you play that, play the sequel, The Great Forever Tommorrow. It's more linear but the graphics, levels, and presentation are improved. There are new weapons, more colorful levels, and slightly deeper plot:
http://www.planetphillip.com/posts/the-great-forever-tomorrow-half-life-2-ep2/
Both mods were developed by Kiwi Productions.