Windows only: If you need to prevent an application from running accidentally or would like to disable an application you can’t uninstall, AppLocker provides a simple interface to do so.
Whether you’d like to prevent Internet Explorer from running because you can’t uninstall it to get rid of it or because you’d like to disable web browsing on a computer used by a young child, AppLocker allows you to create a black list of applications that cannot be launched. The program isn’t as robust as making a separate user account and setting security policies but for a simple fix to keep a group of applications from being launched it’s more than adequate. AppLocker is freeware, Windows only.
Previously mentioned ad blocker
The Associated Press gets a bit, well, blog-like in an article detailing 20 recurring fees that seem like unavoidable annoyances, but can be worked around with planning and a strong dose of Won’t Stand For It.
Linux only: Reader Chris writes in with an excellent tip that changes the prompt to red when using the root account from the terminal—as a reminder to be more careful.
Lifehacker readers 
Windows XP does have the ability to format drives with the NTFS file system, but you wouldn't know it by looking at the format dialog—normally the option is disabled. To enable it, open up Device Manager and find your USB drive, go to the Properties -> Policies tab and then choose "Optimize for performance". Once you've done this, you'll see the NTFS option in the format dialog.
Web application Namechk finds available usernames or vanity URLs on more than 80 web sites—so you can own your online presence.
Firefox: Thanks to an explosion in their popularity, it’s hard to avoid the ad-filled countdown screens of file services like Rapidshare and Megaupload these days. Skip the wait with this handy extension.
Open Culture has a comprehensive list of totally free resources for anyone looking to learn a language though audio files. Whether you’re brushing up your Yiddish or delving into Dutch, you’ll find a free feed here.