Frederik Seiffert provides this nifty tool, LastHistory, to visualize your Last.fm listening history. Mouse over songs and find repeated track sequences. The visualization itself isn't all that useful, but it gets interesting when you hook your calendar and photos in with music. LastHistory lets you replay songs synched with your photos, and your slideshow suddenly gains a new dimension.
I don't use Last.fm, so LastHistory isn't much use to me, but I'd be interested in hearing what others think. Give it a try for your self though (for Mac only), and leave your thoughts in the comments below. Is it a reason to start using Last.fm?