Half-Life: Invasion is cool if you've beaten all the half-life games on difficult mode, cause it's a real challenge. Be sure to use the latest patched version, and also be sure to use my replacement models that make the low definition grunts look like high definition ones!
Favourite mod ever: Sweet Half-Life. That mod is pure genius. And, I beat the boss fights without cheating, but that's cause i'm just so damn tough.
Other favourites: Azure Sheep and Point of View, Heart of Evil...
oh, don't forget the Timeline series! Timeline 1, 2 and 3. The first Timeline mod is ok (Neo-Nazi grunts in Black Mesa travel back in time and give advanced technology to Nazis, changing history). The second is genius (parallel Earth in an Ice Age, where Britain never lost control of America, and you visit London infested by Xen aliens!). 3 is incredible (a ride through time with references to HG Wells' the Time Machine, as well as visiting Black Mesa before the disaster took place.)
Play all three, one after another, for total Half-Life satisfaction!
I'm going to have to check that you can still download all the timeline games, cause just talking about them makes me want to play them again, lol!