SMOD a japanese mod for HL2 with lot of fun weapon.
Scisosr, stuff gun, Singularity bomb, strider cannon etc
Random ennemies when running the game and new ennemies (stronger CP, Jumping Combine-soldier)
most of the mod is better handled via the console .
Cinematic Mod is my second favorite, but it need a lot of memory to run (3 to 4 GB)