Recently Humble Bundle implemented direct Steam connection, check it out here: https://www.humblebundle.com/user/settings
Activates Steam Keys in bulks (Mac only)
It's a small and dumb script that reads the currently open Safari/Chrome page, searches for Steam keys and uses local Steam client to activate them one by one.
It's particularly useful for Humble Bundles.
How to run:
1) Check that you are using Mac OS and not Windows or Linux
2) Download everything to your Mac
3) Run this application and it should work just fine
Known to work with:
1) Humble Bundle 6
2) Steam in English September 2013th version
3) Mac OS Lion
Good luck! Feel free to send patches and bugs!