the first one is blender source tools. this will probably be the most used one. it lets you import .smd, .vta, .dmx, and .qc files. it also lets you export them. this isn't the only addon that lets you do this, but it's the most popular.
the next one is io_import_vmf. it lets you automatically import textures from source games just by importing the .vmt. it also will create a node set up with the textures; all you'll have to do is apply the texture to the model. it will also let you import .vmf, to import maps with options to import textures and props.
the final one is sourceops. it lets you generate .smd, .qc, and .mdl files, all in blender.