Well, if you are going for a single-player experience, definitely get They Hunger 1-3
http://en.wikipedia.org/wiki/They_Hunger. Also, before he made that, he made a map-pack for Half-Life called "USS Darkstar", also worth getting.
http://en.wikipedia.org/wiki/USS_DarkstarAs for multiplayer, my favorites were Science and Industry
http://en.wikipedia.org/wiki/Science_and_Industry and Front Line Force
http://en.wikipedia.org/wiki/Frontline_Force. I never played online to much with Half-Life because I had a 56k modem for the longest time; those two, however, were really fun.
There was also Holy Wars, which was basically felt like Quake III Arena on HL, due to the bulky models and arena intentially being designed that way. It had a whole bunch of newly-deceased people fighting in purgatory over a Halo that would allow them admittance to heaven. If someone got the Halo, then it was all against that one person. If you kill someone without a halo while you aren't holding it, you are branded a heretic and suddenly you also become a target for everybody. The Halo would also grant the holder regen, high defense and quad damage, so it would require everyone to drop their rivalries and form quick alliances to take down the halo user before they held it for enough time to win.
There was also another mod I played that was really interesting, but I can't remember the name; You only had 5 weapons, which were modified version of existing HL weapons (The Crowbar was an insta-kill, the Machine Gun had infinite ammo, The Shotgun became a Sniper Rifle, the Rocket Launcher became a devastating Mortar and the Trip-Mines became Proxy Mines), plus everyone had rocket packs that would activate just by holding jump after jumping. The best part, though, was that the mod was compatible with any Half-Life DM, Counter-Strike or Team Fortress Classic map. If played on a CS or TFC map, it went from an insane Deathmatch game to an insane Team game. It made the game truly epic.
