Half-Life, in my opinion, is one of the greatest PC games of all times. If you liked the game, then you should probably check out Black Mesa, a remake of the game using Valve's Source engine. This has the effect of essentially playing the same game with a totally updated atmosphere, making a great old game look more up-to-date and adding a few new features in the mix. Find out more or download the game (free of charge) here:

http://www.blackmesasource.com/