Half-Life Visitors

Half-Life Visitors is a single-player modification for Half-Life. The game's story puts you in the shoes of Alan Green, a lawyer (although the website's story mentions you are a Doctor) visiting the Black Mesa facility.

You are heading to the visitor's area but there are just some malfunctions in some systems along the way. So you are heading for the cafeteria when Freeman's experiment goes wrong. You pass out, and you wake up with monsters all around. Your only goal is to get out of the facility by any means necessary. Unfortunately, your only routes to keep moving take you further underground, where you find more than you should know.

The modification's story is very creative, as we hear there are visitors in the facility on the original Half-Life. Even the character's skins are changed to reflect this. The game is entertaining and the levels are challenging.
There are new weapons, as well as new enemies. Considering all this, this mod is worth your time.