Of course "cool" is very much in the eye of the beholder, but the ones that make me smile the most are below...they also stand out in that the first three are the only third-party apps that my wife (Mrs. Perpetually Tech-Unimpressed) actually asked me to install on her Pearl as well...
1. BerryTunes - internet radio and podcast streamer...allows me to listen to radio and my favorite podcasts anywhere I am. Can't live w/out it.
2. Google Maps & Windows Live Search - tells me where I am, how to get to restaurants, stores, etc., and what movies are playing when/where. Street view in Google Maps on a BB has to get a very big "COOL!" vote.
3. Vlingo - voice-to-text app that lets me "write" emails and texts by talking to my BB...requires some correction before sending, but fixing a few words is a lot faster than typing a whole bunch of 'em.
4. Wattpad - doesn't look like much, but this is a simple ebook reader that allows various font sizes/colors and has adjustable auto scrolling for very good readability for me. A surprising number and range of books available.
Sorry - nothing too flashy, but way cool in my book.