MacUpdate is offering Little Snitch from Objective Development, a beloved 'reverse firewall' for Mac OS X, at almost HALF-PRICE this weekend. That's $14.99. (Regular price is $29.95) Go get it here:
I use it non-stop and love the thing. It has gotten consistently easier to use over time. I also have an older version running on my FTP server 24/7. If you're worried about being pwned, this will stop all communication from malware back to the Bot Wrangler. No botnet for you! It's also perfect for stopping all 'phoning home' by pesky applications.
MUPromo's offer drops dead at midnight on Sunday, January 30, 2011. But you may be able to get it for a lower discount during the following week. Check the MUPromo website for details.
Note: If you use the 'reverse firewall' in Intego's VirusBarrier v10.6, you don't need Little Snitch.