Black Mesa Sideline is a small Half-Life singleplayer mission. It contains 2 maps and is fully compatible with HL-Steam and HL-Won.
Its one of those very rare, excellent short mods (~15 minutes), it has beautiful level design, attention to details to the texturing, pipes, metal lining along the bottom of walls. Unfortunately it provides very little variety, with 2 very small outdoor areas in between the action, and one 2 tight air ducts (one of which had a very nice scripted sequence, THANKS BARNEY!!) this level design didn't feel claustrophobic because there was ample light and grates on the walls to see around. The level design is just tight, which doesn't do well for the combat IMO, these sorts of sequences would have been great to add flavor to a bigger mod... but with ~15 minutes it paints a very average picture.
With no puzzles or storyline it all rests on the enemy placement and level design.
The difficulty is fairly average, the hardest part is the beginning with the 5 grunts, one oddly placed satchel charge that can put in trouble if you rush forward passing it and the standard 4 weapons with some ammo is a overall average execution.
If you are on coffee break its worth playing.