http://cinematicmod.com/cm_10.php
It replaces a lot of textures and models in HL2 with very very high quality alternatives. Tons of super-crazy high-res textures. It also brings all HL2 games into the latest version of the source engine, which means HL2 and EP1 get all of EP2's HDR and dynamic lighting/shadow upgrades. Actually, the PS3 and 360 Orange Box versions of HL2 and EP1 actually run on the EP3 engine as a matter of packaging simplicity, so they get the HDR and shadow upgrades while the PC versions got left out.
It's really demanding, though. Years ago my machine was good enough to run it, but the mod has been progressing steadily over the years and it eventually outran my old rig. Now that I've got a modern computer again, I figured I'd give the latest version a go. But the official site's torrent file does not work, and I'm not up for downloading 47 200MB parts from rapidshare.
Part of the reason I want the latest version is that the improved model for Alyx has switched from the goofy looking Adriana Lima lookalike back to a high quality version of Valve's own look, which is much, much better in my opinion. But it also has a lot of other upgrades the older versions don't have. I can find links for 8 and 9 on fileplanet, but I really want version 10 for the extra goodies.