One of the great progressive rock bands, although on the softer, moodier side, is The Moody Blues.
Back in Season 2, the December 7, 2008 edition of The Irrelevant Rock & Roll & Music Show included a Retro Retrospective on The Moody Blues. You can listen to the show here.
For more on The Moody Blues, check out their website.
Another great source of info and commentary about The Moody Blues and their extensive musical legacy, is their page at Progarchives, the Ultimate Prog Rock Resource.