This one's a bit obscure and of questionable legality, but too good not to mention. I just stumbled upon
2DF FreePlay, essentially a matchmaking program for emulated arcade games.
Which means, if you don't see why that's such a big deal, that I now have an arcade-perfect version of Street Fighter III: Third Strike* with decent online play (I got used to doing quarter-circles with the numpad playing MUGEN, so controls aren't a problem). As I understand it, the Xbox version a while ago had crappy netcode; this is using some manner of P2P hosting that seems to work quite well.
The nice thing is that most of the people I'm running into online don't seem to be of the super-hardcore-impossible-to-beat type (those are probably playing some private ladder matches or something). They're also, bizarrely for games focused on hitting people in the face, nicer than most online communities. One guy spent several rounds instructing me how to use Makoto's special throw after he realized I was missing opportunities for it (I used it on him a few times after that, and won our last fight).
There's a few steps to go through to install it, but finding the roms (if you don't have them already) is as simple as throwing whatever filename it says it needs for that game into google and then telling the program what directory you put them in. My only real complaint is that nowhere was I told "T" was the chat button, which I had to find out through trial and error.
Anyway, if you have any interest in classic fighting games, I'd say it's worth the installation kerfluffle. There are other games on the service besides SFIII but really, fuck them**.
* Which is a fantastic game, and this coming from someone who doesn't even
like Street Fighter for the most part.
Makoto is the best fighting game character ev4r, just because if you're feeling mad during the match there's nothing more satisfying than doing her grab and watching her choke-slam the other guy, then try to put her fist right through his abdomen while he's still on the ground.
** Except for maybe Last Blade 2. That one can remain un-fucked as well.