Thanks to a comment from Ryan (see comments below) I'm now aware that homebrew itself has a homebrew-dupes repo.
From the README:
Just brew tap homebrew/dupes and then brew install <formula>.
If the formula conflicts with one from mxcl/master or another tap, you can brew install homebrew/dupes/<formula>.
You can also install via URL:
brew install https://raw.github.com/Homebrew/homebrew-dupes/master/<formula>.rb
Updating mac homebrew from 0.7.1 to 0.8 removed the rsync formula that I use to update mac rsync from the supplied 2.x version to 3.x.
This is due to the fact that homebrew doesn't want duplicates of existing software.
However - these are still available via homebrew-alt
For example - to regain the rsync formula was as simple as:
brew install https://github.com/adamv/homebrew-alt/raw/master/duplicates/rsync.rb