Half-Life and Half-Life 2 have plenty of really good mods if you are interested in those, Black Mesa started as just a mod as well.
Some of my favourites for HL2:
- Entropy: Zero Two (on the same level of quality as Black mesa and its free on Steam)
- Nightmare House 2 (now named Nightmare House: The Original mod)
- Half Life 2: MMod
Some of my favourites for HL1:
- Half Life: Field Intensity
- Half Life: Delta Particles
- Half Life: MMod
There are many more that I haven’t mentioned / played myself.
I like boomer shooters and immersive sims, so I picked up Selaco and Fallen Aces. Both are good so far. Selaco has some maze-like levels that can be a bit annoying, but the gunplay is very fun and the level of polish and detail is very high. Fallen Aces is awesome for the amount of ways it gives you to deal with the goons.