MessageFont is available now on the Mac App Store:

If clicking the above link does not automatically open the Mac App Store, perform the following:

1. Choose App Store… from the Apple menu.
2. Enter MessageFont in the search field.
3. Click on the MessageFont icon when the search results appear.

Note: You must be running Mac OS X 10.6.6 or higher in order to have the App Store application on your Mac.

Trial Version of MessageFont

Want to try MessageFont before you buy? Stay tuned, a trial version is coming soon.