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:
Some of my favourites for HL1:
There are many more that I haven’t mentioned / played myself.
Steam DB is useful for steam related stuff.
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.