+Willem van de Beek
It's not in the standard repos, but there's a PPA available for it:http://ubuntu-tweak.com/downloads/
You have to go into either the Software Centre (it's under the edit menu) or the Software Updater (it's the Settings button) to access the dialog for manually adding repos in elementary, but other than that you should be good. It also doesn't seem to appear in the app menu when it's in grid view for some reason, but if you switch to Category view then it's under "System Tools".
Once it's installed and running you want "Old kernels" under "System" in the Janitor tab. Simply select the kernels you no longer want and hit the "Clean" button in the bottom right. It's smart enough to not display the current kernel, so it should be safe to use the "select all" check box, but if you want the safety of having a backup kernel then select everything except the most recent kernel in the list.