The only mods I know of for half-life are blue shift and opposing force. I don't think they work any more because they are their own separate games.
u wot m8, OpFor and Blue Shift are
expansions, not mods. They work perfectly fine, but I think they install Half-Life to run off of (which you won't be able to run if you don't have it).
As for Half-Life mods, there's a ton. The aforementioned Sven Co-Op, Natural Selection...the list goes on. There's a whole bunch of still-active mods out there that you'll miss out on with Source...though you do get the resources for Gmod if you buy Source.