git config --global alias.home 'rev-parse --show-toplevel'
Now, git home prints the full path to the root directory of the current project.
To go there, type (Mac/Linux only)
cd `git home`
Notice the backticks. They're not single quotes. This executes the command and then uses its output as the argument to cd.
This trick is particularly useful in Scala, where I have to get to the project root to run sbt compile. (Things that make me miss Clojure!)