Some good mods include Synergy, which allows you to play up to 10 player Co-Op over the internet, Research & Development which is a very clever puzzle mod. Dear Esther is also an interesting mod, but its a lot more for its subtextual commentary on level design more than action or discovery.
A very good webcomic was made a long time ago which is now Legendary among Garry's Mod webcomics, which is Christopher C. Livingston's Concerned [http://www.hlcomic.com] which is a very funny demonstration of the whole Half Life 2 game.
Additonal recommended watching are these [http://www.youtube.com/watch?v=OHxyZaZlaOs] dumb [http://www.youtube.com/watch?v=h8yHguvYYyQ] videos [http://www.youtube.com/watch?v=ULWgEnnmcv4], the infamous Parody [http://www.youtube.com/watch?v=fOvglodUIcA] Fan-fictions known as Full Life Consequences.