While I could, in light of recent events, recommend the map "Experimental Problems" as the next in line for you to try out, I'm not going to. Instead, to celebrate the comeback or whatever you call it, I'll just list ten really, really good HL1 mods here for you all to try out.
Azure Sheep
Cthulhu
Heart of Evil
Night at the Office
Operation Black Thunder
Point of View
Poke646 and Poke646 Vendetta
Sweet Half-Life
They Hunger
Todesangst 2
And here's ten really long mods of varying caliber.
Black Death
Chemical Existence
Dark Territory
Hour-glass (yes the one with the inappropriate content)
Intolerable Threat (need OPFOR)
Invasion (the French one)
Residual Point
Survive in Catacombs series
Xen Assault
Zombie-X
Most of them are available on moddb, I think.
Tell me what you think of these.
poke646 - best mod, hands down
sweet hl - pretty good, too many anime references, terrible voice acting for hwg.
they hunger - pioneer of half-life modding (although USS darkstar was underrated as hell)
residual point - pretty epic mapping.
I never got through Azure Sheep. I could never see what the big deal about it was. Did it have a really great story to make up for the incredibly inconsistent and at times juvenile mapping? I have no idea, it wasn't comprehensive enough to hold me in and intrigue me. When it came to the point where entire maps consisted of nothing but endless empty hallways that made use of an entire three textures, I gave up.
Azure sheep was okay, if a bit nonsensical at times, (Shepard and Freeman both make a non-canon appearance and sound the same)
And They Hunger was 25 karat gold, if a bit low-poly. Haven't gotten a chance to play anything else on that list.
as for they hunger, I played all of it on svencoop, very fun when playing it with many other players.