I've been playing HL2 mods a lot lately. They're simply amazing.
Substance mod: Switch charactors with totally different weapon sets and abilities on the fly, new effects, 3-4 different gameplay modes. It's like 6 games in one.
Causality Effect: A Red Letter Day Gone Wrong: Amazing story line thought up and done by ONE guy. Amazing work, very creative stuff. Inspired by the movie Donnie Darko.
Eclipse: This has to be the most extensive total conversion mods ever done for HL2 that I know of. Really nice gameplay, very creative story line, stunning artwork. Definately something that shouldn't be missed. Even if the genre of the mod doesn't float your boat, the extensive dedication this mod must have required is something everyone can appreciate.
All HL2 mods can be found at Moddb.
DoomsDay engine for Doom Engined Games: This is simply awesome. Every game that ran on the Doom engine can be run thorugh this engine, and everything is 3d accelerated. Stunning graphics when compared to their native counterparts. Google Doomsday and it'll be on the first page.
Duke Nukem for Windows XP. Not sure where I got this from. It's similar to Doomsday, but it's for Duke Nukem. Enables high resolution and 3D acceleration. Keep an eye out for it on torrent sites. All you need is your original Duke Nukem files and presto, "Let's Rock".
The GTR Tuning Mod for GTR Real Racing is a must have. Every car has been reworked and turbo's added. The Viper has been turned into a beast with over 1000 ft/lbs of torque. It'll spin the tires in 3rd gear with TC on HIGH! Every car is a work of art. Mod can be found at GTR4u.de. German site, but menu is in english. Look to the right and the mods will be there.
The V8 Supercar mod for GTR2 is
flat out insane. I've never experienced closer sim racing in my life. If you want a racing experience that'll make you sweat, this is it. Really intense competitive stuff here, all cars are equal and feel
fantastic on the limit. This can be found over in the mod section over at RSC's website.
Enjoy!