Thanks for the tag lux.
You can download it here and find ridiculously thorough documentation of the changes as well:
http://balancedbrawl.net
The basic design philosophy behind Balanced Brawl is that it aims to play fundamentally like Brawl while fixing the various balance problems within Brawl. This involves balance changes across the cast as well as several systemic changes to fix some basic gameplay issues (like an anti-planking code, a few small shifted mechanics to get rid of all known infinites, and some stage changes), but it doesn't aim to change the basics of how Brawl works so it leaves the basic dynamics and physics of Brawl in-tact. Even on a per-character level, we endeavored to maintain as much of any given character's fundamental playstyle while moving that character to a well balanced place; we had to get creative on a few characters like Captain Falcon and Ice Climbers, but by and large, we feel like most if not all of the cast plays very much like they did in Brawl except more balanced. Note that balanced to us means the characters must retain unique and diverse gameplay, be equally viable, and be non-polarized with each other (that last point means all individual match-ups should be as fair as possible; it's not okay if Samus destroys Bowser and loses badly to Fox as her avenue to "balance", for instance).
Thinkaman and I develoepd it as a pair without significant outside assistance (we used coding resources others had made but did 100% of the design and particular implementation ourselves). In all of our testing and the feedback we've received, we got the balance pretty close to right (and we did balance pretty high on the balance curve, approximately to where Marth was in Brawl). Unfortunately, we focused all of our efforts on development and very few on marketing, and we ended up bungling the marketing side of it really badly and never caught on. We ended up reaching a point where we had the game as balanced as we could get it without substantial Balanced Brawl touranment data, and that never came so we declared the game done.
I do remain very proud of the work we did there and firmly feel it is the ultimate version of Brawl, and if you have any particular questions about specific things in Balanced Brawl, I'd be more than happy to answer.