RCDefaultApp for OSX
Quite simple. As it says on the tin. OSX will often try to "intelligently" associate your files with the software most likely to be able to open it. But this is not what you always want.
For instance, I do not want *.png files opening up with Adobe Fireworks just because I use the software to create new ones.
Here's a mighty useful app that allows you to set the default applications for file extensions, or mime type, or most common uses (such as "Email", "Browsing") etc. It sits snugly within the "System Preferences" and is quite easy to access.
How it looks:
Click on the image to go to the developer's website.