Some people maybe never played the original Half Life.
A 40-person volunteer development team worked hard (Eight years development) and released Black Mesa about 2 weeks ago. Black Mesa is a remake of the original game using the more advanced capabilities of the Source engine. Its standalone so all you need is the Source SDK 2007 available on steam.
Download here http://www.blackmesasource.com/download.html
22nd [SAS] Elite Virtual Regiment