So everyone knows of Half-Life 1 and 2, but not everyone knows of the two offshoots of the first, Blue Shift (which was originally a mod) and Opposing Force.
There is a large group of people who have played Opposing Force and hail it as the best Half-Life game made thus far. It really recreates the whole "all on your own" feeling that Half-Life 1 gave you, without being too much of the same slog.
You start off as heading into Black Mesa as one of the military specialists assigned to eliminate Gordon Freeman. You fight against aliens, and the special ops (who I originally thought must have been ninjas), later on you have to get by tanks and other large, pain in the ass enemies.
The basic arsenal is pretty much the same as Half-Life 1 but this game has some cool and unique special weapons, some of which I haven't seen any other game try and reproduce, but are awesome without question.
The level designs are great, and it's a little quicker to get right into the action than the first Half-Life (no wandering around corridors, picking up your suit (which for some reason seems to engage an MP3 player), wandering around corridors, waiting for scientists to open doors, pushing a cart into a machine, etc. etc. For me the first Half-Life was great, the beginning was just so bloody tedious and this one cuts that out.
This game got named "Game of the Year" by the Academy of Interactive Arts and Sciences, and I would definately recommend checking it out. The 4.99 on Steam is well worth it.