Fightcade is a matchmaking client that includes an emulator and allows people to play each other online using the very well respected GGPO netcode. It currently supports nearly 200 classic arcade titles, including many fighting games. Each game has its own room where players can challenge each other, spectate matches, or just hang out and chat.
Why should you check it out?
For many of the games supported on Fightcade, it is the best way to play them online on PC. There are other online capable emulators, but they do not offer a comparable netcode to GGPO, have active communities, or have the benefit of being extremely easy to set up. And in some cases, even the official releases aren't as good as playing on Fightcade.
What is so good about GGPO?
GGPO used to be a service like Fightcade, but now it is just a great rollback netcode. While I can't explain the technical side of how a rollback netcode work, I can say that this one tends to offer the best online experience and is considered to be the gold standard of fighting game netcodes. The devs for Killer Instinct and Mortal Kombat XL used GGPO as their inspiration when creating their netcodes, and as a result the online play in those games is amazing. Skullgirls uses GGPO, and GGPO has become a popular choice for official ports of classic arcade games on consoles and Steam.
How active is it?
The easiest way to answer this question is with this link. I will say that the most active games at the time of writing this are KOF 02', KOF 98', SFIII: 3rd Strike, Super SFII Turbo, and SF Alpha 3.
How much does it cost and how do I get started?
Fightcade is 100% free*. Just register a new account and download Fightcade from here. You will need the games to play, but this is not the place to discuss that. My recommendation is to go into a room in Fightcade and ask the good people there for help. Or you can PM me about it.
Why are you telling me this?
I just think it's a great service that anyone who enjoys classic fighting games and other arcade games should be checking out.