Mac: NexTab is a handy utility that lives in your menubar and adds some really nice functionality to your trackpad. If you're in any tabbed browsing application, whether it's a web browser or not, you can move left and right through tabs with just a tap.
Once you open the application you're all set. You just place on finger on the trackpad and tap with a finger on the right or left side of it. When you tap, nexTab will switch to the tab on the side you tapped. For example, if you placed your middle finger on the trackpad and then tapped with your pointer, you'd move one tab to the right.
While it's easy enough to cycle through tabs with keyboard shortcuts, when you're using the trackpad for scrolling or anything else it can be really nice to be able to do the same thing without too much movement. Although not necessary, it's an elegant little time and effort saver that makes browsing a bit easier.
nexTab ($0.99) [Mac App Store]