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
• 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.