[Little Snitch](http://www.obdev.at/products/littlesnitch), a Mac utility that monitors and control network access from a computer, is on sale for 50% off for 47 (more) hours. I find Little Snitch to be absolutely indispensable, and feel naked using a computer without it. I rely on Little Snitch to tell me which apps are requesting connections to the internet, and approve/deny access appropriately (you can set permissions for the session, or for all time). It's useful as a monitoring app, but I've actually found it to be the most useful when I'm traveling and using low-bandwidth internet connections. In these cases, I tell Little Snitch to block ALL internet access and open specific pipes for whatever I am using at the moment (usually, email, twitter, and web). This prevents your computer from going online and, for example, downloading OS and application updates.
Little Snitch is
on sale via [MacUpdatePromo](http://www.macupdate.com/app/mac/10426/little-snitch) no longer on sale.