Is it really a sin to listen to mainstream music?

Well not at all, the music that I take a liking to just happens to be popular.

No. The majority of the music on my ipod is indie pop, bands like Camera Obscura, things like Animal Collective, as well as hip hop, The Wave Pictures, folk and rock, some jazz...

People always scroll through the artists and pick out something like Brandy or Britney Spears or Rihanna and laugh a little and I tell them to shut up.

If you like mainstream music, then that's good. Don't change what you listen to because someone decides that it's not cool or credible enough. I'd rather listen to Girls Aloud and The Saturdays on repeat than Neutral Milk Hotel and Of Montreal or John Cage and John Zorn or whatever just because I thought cooler people would like me more because of it.

Just because it's mainstream shouldn't stop you from liking it.

Then there's no problem; though if you consider yourself a true aficionado (rather than a dilettante) then, at the risk of seeming trite, I'd recommend broadening your horizons: there are better bands 'off the radar', as it were.

I'm trying Where do I look to find these good 'off the radar' bands? Because the only way I've tried so far is look at stats and charts, and the occasional visit to the online radio but most of them as you can imagine only provide the mainstream popular artists. I do occasional discover some of them at sites like last.fm however.

Start perusing Last.fm in earnest; 'artist radio', 'tag radio' and 'personal radio' are all abundant sources, especially if you have some idea about genre and are able to exploit the 'audioscrobbler' plug-in to the fullest of its potential.