Osx-package-manager-update-notifications is a project mainly written in Shell, it's free.
Shell + growl scripts for notification of updates for various package managers on OS X.
These scripts check various package managers for updates and informs you if there are any available via Growl. There is currently support for the following:
There scripts are can be invoked with launchd to allow them run at startup or on whatever schedule you like.
In addition to the package management systems, you'll need to have:
Put the scripts somewhere on your system and make the executable.
If you want them to run automatically, use launchd. A few example launchd configs are includes; you can drop them into ~/Library/LaunchAgents (or /Library/LaunchAgents if you want them to apply to all users on your system). You will need to update the paths to point to wherever you placed the shell scripts.
Right now, the paths to the package managers (brew, gem, npm) and growlnotify are hardcoded; you may have to edit the scripts to point to the correct location on your system.
Another thing to watch out for is that the gemupdates.sh script assumes the system copy of gem; I'm not taking into account something like RVM here yet.
If you want to simplify setting up launchd scripts, take a look at Lingon.