Compilations are everywhere and are a convenient way to get your music out to a public that doesn't even know you exist yet. There are many, many people looking to put out original music by underground artists, you just have to look around for a while to find them, as it may always be happening seemingly just out of your reach. Websites and blogs that promote music are often putting together, or considering putting together, compilation albums for cross-promotion purposes, or simply to give others the chance to hear their favorite artists... or both!
There are a lot of artists I got into because I heard them on some comp. I picked up somewhere, whether on the internet or by hard copy. In fact, oftentimes I end up losing my CDs but I can still remember artists off of the albums who really blew me away. Right off the top of my head I can think of several artists who I started listening to after hearing them on comps. I first heard EPIC, Lexington, Big Ref, and others from the Hokey Religions Mixtape, put out by Hand'Solo Records (find them on the right toolbar) up in Canada. I first heard Metamystiks Incorporated, Futuristic Sex Robotz, MechP, ytCracker, and many, many more from the numerous free compilations RhymeTorrents.com (Right toolbar, again) has put out over the last few years. And surely, if I had not been a part of the RT community back when I was nobody would have ever heard my music, I wouldn't have started Legendary Hip Hop, and I probably would have never gotten into hip hop writing.
One website in particular has released a slew of compilations, constantly finding new approaches to what could seem to be overrated topics and pushing the limits of compilation collaborations. And the kicker? It's all [except one album] free, and all non-profit (I know, I talk to the man behind it: Genoboost). Game Music 4 All (Right toolbar!) put out a fantastic compilation called Here Comes a New Challenger last year, pitting chiptunes artists against nerdcore hip hop artists, bringing together quite the slew of nerdy musicians from both sides of the tracks for what can only be described as an epic listening experience. Taking chiptunes out of their element by forcing them to turn their style into something more rappable and making rappers spit over something completely different than normal (for most of them) led to many interesting tracks, as well as some of the greatest artwork I have ever seen. It's work like this that really makes communities come together.
And guess what? Here Comes a New Challenger Volume 2 is now in production, though the open submissions have closed, which allowed many artists who have not yet had any official releases a chance at some exposure, which may be exactly what you're looking for.
Honestly though, nowadays, with the internet serving as a huge network for musicians, it's easy to find something going on, or at least set yourself up to find something. Utilizing MySpace as a networking utility for music work is easy; all you have to do is look around at artists you're interested in, link through their friend lists and influences... but that's another article entirely.
Just get out there, find compilations to get on! If you have compilation albums, look at who put them out! Dont' forget, you can always make a demo CDR and mail it out to people who have put out comps, urging them to consider you for their next album. Consider the fact that when you get on a compilation with 15 other artists, all of their fans who listen to the album will hear you. It's one of the greatest networking tools, and that's why it exists. Do what you need to do. Find compilation albums and get on them.
Spread the music.