I've never played Half Life, but I guess it's somewhat similar to Counter Strike type games. First person shooters. Some of my friends get together from time to time to play CS for a bit. It's low on resource requirements, so can run from most computers.
StarCraft is similar in a way to C&C Red Alert, etc. Where you command your army and have a view over the whole battlefield. Playing SC at battle.net is probably the best. It's a great game just by itself, but online there are so many unique maps and ways to play the game. It was a lot of fun back when I used to play it. Too much fun, actually. I deleted that game off my computer probably 4+ years ago as I was having too much fun with it and not getting enough work done.
