For those who don't know Black Mesa is a community remake of the original half life. (it's $19.99 on steam check it out or wait for a sale. It is currently an early access game, the only major thing that's missing, that I know of, is the very end.) Although I have yet to try it out myself it has multiplayer.
If you have played what is currently out what is your favorite glitch?
What is the most frustrating part?