I make extensive use of http://twitter.github.com/bootstrap/ (a godsend for hackers), though I use it more as a style-guide and rebuild most parts myself.
The ui/ux evolved in this app, which means I started with the bare minimum requirements (it looked bad. really bad) and continually improved the "look & feel" as needed. No photoshop involved.
It's still not finished, but it has come a long way :)