There were two expansions too Half Life (review, story synopsis) made, both by the outside developer Gearbox Software, Blue Shift and Opposing Forces. Both games are moderately short and take place at the same time as Half Life and like Half Life as set within Black Mesa and the alien world Xen. Additionally, they both have negative and positive aspects and while they overall hold the same Half Life feel play very differently.
Blue Shift added a High Definition pack that upgrades both Blue Shift and Half Life, if you so desire (note: it is not the default and it actually rather tricky to get Steam to even do it). But other then this it pretty much relies on content from the original Half Life set within new levels and challenges. In this game you play the security guard, Barney Calhoun and what I loved the most about it was the how tactical it often got while you were fighting groups of enemy soldiers, requiring intelligent use of grenades and remote controlled bombs to prevent you from being overwhelmed (which I found was not necessary in the original Half Life).
Opposing Force adds a bunch of weapons and removes a few others from the original Half Life, overall making your entire arsenal usable and far better then the original. In it you play Adrian Shepard, a US marine and one of the enemy characters in the original game (though not specifically). The best part of this game is, by far, the new and powerful weaponry, but one of the bigger features is the addition of the ability to control a squad of soldiers, engineers, and medics who help during battle and in the case of engineers are sometimes needed to progress. Unfortunately, these squad controls are also the worst implemented aspect of the entire game and are not even adequate in simply getting all of your squad to follow you around.
Both of these games were a pleasure to play and both are great in there own way and I would recommend them to anyone.