Today, I found out about the band Caspian! It's great! Their songs are instrumental, introspective, something like a crossover between progressive rock and ambient music.
Caspian was formed in 2003, in Beverly, Massachusetts, US, and is composed of Philip Jamieson (guitar, programming, samples), Calvin Joss (guitar), Erin Burke-Moran (guitar), Chris Friedrich (bass) and Joe Vickers (drums).
About the unusual choice for being an instrumental act, Philip said that “We initially wanted to have a singer, yes. We started Caspian to be an outlet of free expression, without the confines we had felt in previous bands, so having a singer wasn't a necessity though. After a few shows instrumental we realized we had found a way of communicating our thoughts and ideas that felt pure to us as a band and we decided to go from there.” 
Philip also described the band’s sound as “Caspian is an atmospheric, instrumental rock band. Our music is intended to be powerful, epic and emotional while trying to avoid the musical cliches that are often associated with those kinds of descriptors. We are heavier and more aggressive than most instrumental "post-rock" bands, without being metal. There is a lot of light and shade to our music.” 
Check this clip of their latest album Tertia, and visit their myspace page for more info.