First, make a list of your top-20 artists overall.
Then, for each of these artists, add the 8 most similar artists (according to last.fm) to your list. Delete any duplicates, count up the number of entries on your list and this will give you some idea of how eclectic your listening habits are. A score of 9 represents an extremely unvaried musical taste while a 160 represents an extremely varied one.
Total original artists = 134
The J's are original to the core, all 8 similars stay, and I highly dig Shootys Groove, Jimmy's Chicken Shack and Outkast.
Say Anything surprisingly also keeps all 8. The mantra of critics that "they sound like everythingelse" apparently does not hold true, and I would agree.
Splender kept all 8, but that's not a shock since I'd be hard pressed to explain what they really sound like. One of the most under-appreciated bands I know of.
Mike Doughty kept all 8 similar artist, with 6 that I've never heard of.