General Bra Fitting/Calculators
Bratabase A database of over 20,000 bras. Includes measurements and user reviews, as well as a blog. Also, there is a forum for requesting fit help and a bra sale/exchange area. Overall an amazing site.
Sophisticated Pair Fitting Calculator Also check out their blog and their store!
A Bra That Fits Reddit Great advice about sizing and fit, tons of links, and a sale/swap board.
Good Places to Shop (small business)
Butterfly Collection They also offer Skype fittings. And check out the blog!
Good Places to Shop (larger retailers)
Bare Necessities US-based
Bra Stop UK-based, great discounts