Blackmesa Fallback

This is a pretty neat mod.  It takes place in Black Mesa and puts you in the role of the soldiers sent to kill some scientists.  There's a mixture of new content along with textures from Halflife and models from Counterstrike Source.

A friendly chopper attacking hostiles.

The first third of the game has you entering various areas with your fellow squadmates.  There are no puzzles in this section of the game.  It mostly consists of 'missions' that have you opening a door to a building, clearing out an area of enemies, and heading back to your vehicle to proceed to the next area.  There are about five of these missions and I thought they were quite boring, and I was about to quit.  Then the real fun part of the game begins.  It plays quite a bit like Opposing Force, with exploration, puzzles, and fighting new enemies.