I was working on a reply to CrackerSnacker's comment in my music post, but it got so long that I figured it deserved its own post.
Probably you haven't heard of a lot of these bands because quite a few of them are Christian groups (and not well known in the Christian industry either).
For example, Earthsuit is a crazy sounding raggae/rap/rock group that's just a lot of fun. They broke up after their first album, Kaleidoscope Superior. The members formed two new groups, Macrosick and Mute Math. (Listening to Earthsuit or looking at their album art or even reading their new band names is a like an acid trip. Or what I assume an acid trip would be like.)
Burlap to Cashmere is an ethnic Latin/Greek/acoustic folk group. (Yes, I like bands that are hard to categorize) Much like Earthsuit, they put out an incredible debut album (Anybody Out There) that was just too different to get a lot of play on mainstream Christian radio. After a bit of industry drama, their label fell through and the band died out. Steven Delopoulos was their lead singer who now has an amazing solo album called Me Died Blue. If you have any experience with Cat Stevens or other '70's singer-songwriter types, you'll like him.
Relient K is just a fun pop-punk band. Lately pop-punk has become a whipping boy for music critics because it has gained a teeny-bopper following. Relient K is much better than that. Fun music and witty lyrics that really have something deep to say. Their breakout album was the hilarious Anatomy of the Tongue and Cheek. Their latest, which is also great, is called mmhmm.
Rich Mullins may well be the greatest songwriter in Christian music. A good sample of his sound would be the album Songs. He died about 7 or 8 years ago, but had an some unfinished demos for an album called The Jesus Album. They are rough sounding tape recordings that have been remastered to CD, but they're great.
The Benedictine Monks I mentioned are just that, monks doing chants in Latin. Their album is called Chant Noel. It's great music to put on in the background.
Other than that, the only song people probably haven't heard of is Concrete Girl by Switchfoot. It comes from The Legend of Chin their first album (no, The Beatiful Letdown wasn't their first album). A lot of people consider it their best work. I'm not sure I can go that far. All of their albums are so solid that it's hard to pick a favorite.