OmniDazzle is a peach of a program to use when doing presentations, showing off a new feature of some other app, or trying to find your cursor in a sea of pixels (Requires OS X v10.6.8+ but is not compatible with many newer Macs).
OmniDiskSweeper is really great at what it does: showing you the files on your drive, in descending order by size, and letting you decide what to do with them. Delete away, but exercise caution. (Requires OS X v10.8+)
OmniWeb is a powerful, award-winning, feature-rich alternative to the standard web browser. Better organization and more fun with the original side-tabbed browser. (Requires OS X v10.4.8+)
For OS X v10.9, test builds are available.