Underhell is an awesome tactical shooter/survival/story-based, episodic FPS game that always has something new and epic up its sleeve! Highly reccommended.
S.T.A.L.K.E.R. Lost Alpha:
Fans of the game S.T.A.L.K.E.R. Know that the original alpha versions of the game were incredibly different and sported more features and more. This free, standalone S.T.A.L.K.E.R. title is a complete, fully-featured "alpha restoration" based on the alpha versions of stalker based on the playable alpha versions. The entire original and scrapped storyline is available for free!
DOUBLE ACTION: BOOGALOO:**
Ever heard of a Half Life 1 mod called "The Specialists?" If you have, this mod is the spiritual successor to that and then some! If you haven't, the mod is a first/third person multiplayer shooter based on classic 90's action movies like Hard Boiled, Face-Off, etc, with practically the only form of multiplayer-slow-motion that actually works. Kill a dude while diving off a banister and shooting him in the head will get you points. Points earn you a few seconds of slow-motion. Weapons range from dual pistols to shotguns to assault rifles!
MEDAL OF HONOR- BLURRED REMEMBERANCE:
This mod seeks to revamp, retexture, and restore all of the campaigns from the game "Medal of Honor: Allied Assault". The game is essentially a standalone bugfix/mod compilation that had to be extensively reworked to get everything to fit. This means a lot of original content had to be made. The mod features ironsights as well as blood, something the original lacked.
A russian half life 1 mod made standalone thanks to the open-source reverse engineered Xash3d Sourceport of goldsource (yeah, those things exist), this game puts you in the shoes of a Spetsnatz officer, who takes part in a raid of a terrorist-occupied warehouse. Things soon turn for the worse, however, as unknown horrors and scientific monstrosities are found in a bunker beneath the facility...
This UDK game, once a mod for Unreal Tournament 3, seeks to be a Remake/Restoration of the seminal classic, Command and Conquer: Renegade, one of the few First Person Shooters in the C&C series, known for its impact on the genre.
Return to Castle Wolfenstein: Enemy Territory:
Technically a planned-expansion-pack-turned-free and not a mod, this explosively popular online class-based WWII shooter, similar in vein to the likes of Team Fortress 2, is still played today and contains an active community.
Although not a mod, this now community-supported Fully-Featured multiplayer portion of Monolith's F.E.A.R., This title may have a handful of servers, but its fast-paced, nailgun-spewing action is highly-enjoyable to this day.
*Requires Source SDK Base 2007 and a steam account.
**Requires Source SDK Base 2013 and a steam account. Soon to become steam-distributed and may become truly standalone after.