Good suggestions so far and very true that you can do a lot of the stuff you would usually do on a six string. It's a bit more work, but do-able (if the guitar's set up properly).
Early Bowie, most Tom Petty, REM - a lot of more rhythmic song stuff works well.
kansas - dust in the wind. sounds incroyable on a 12 if you can get the rhythm going, and very similar to the dual six-string thing in the original recording.
there are loads of tunes you can play on a 12. but to get the most out of a 12, try some open tunings. man, now i'm lusting.
I had a beautiful Epiphone 12 string for about 10 years.
My favourites were...
Dust In The Wind - Kansas
Stairway To Heaven - Led Zeppelin
Never Going Back Again - Fleetwood Mac (Lindsay Buckingham plays open tunings - I just played normal tunings with a capo)
Wond'ring Aloud - Jethro Tull (hammerons sound awesome on the 12 string)
Busted Down Around O'Connelly Corners - Doobey Brothers
Leaf & Stream - Wishbone Ash (for the life of me I cannot even play this decently on a six string today :-[_