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.
For Macs running OS X v10.4.8 to v10.9, use the download link to the right.
For OS X v10.9.1 and above, use the current test builds.