I'm going to assume that since you're on a Half-Life mod forum, you have Half-Life and can play other mods. So I'm throwing some of my favorite HL1 mods in my list. Good games that don't cost you a cent are hard to come by, but they do exit.
In a vague but not absolute order:
1.) The Ur-Quan Masters (free GPL-port of Star Control II)
2.) Natural Selection (HL1 mod)
3.) ESF (o rly?)
4.) Sven Co-Op (another HL1 mod)
5.) MapleStory (free MMORPG, used to play this one a lot, need to get back into it)
6.) GunZ : The Duel (same as above, minus the "need to get back into it" part [and it's not really a MMORPG, but I just wanted to sound cute])
7.) This (don't you judge me <_<)
8.) Finding loose change in the couch
Sure it's not 15, but I didn't include every other half-Korean half-English MMORPG that's up for beta testing. You can find hundreds of those if you look around, but most aren't worth it.