Way back in 2002, CrazyBrowser released a tabbed browsing wrapper for Internet Explorer 6 and it was amazing. I loved it and used it until I switched to Mac in 2007.
I was still using dialup in those days so I'm of the generation that would open a link in the background tab to load while continuing to read the current page. As such, I hardly used the back/forward functionality and switched between tabs exclusively.
The default keys for Previous Tab and Next Tab were F2 and F3 respectively. This worked out perfectly since I'm a touch typist and have my left hand on the home keys and my right hand on the mouse.
This behaviour has since become so ingrained that I add global shortcuts on my Mac to mirror this as one of the first things I always do when setting up.
However, this necessitates setting F1, F2, et al to act as function keys in System Preferences. This wasn't so bad on my 2006 Macbook Pro since the volume keys were on the left hand side and so was the Fn key. On my new Macbook, the volume keys have been moved to the right and I can no longer hold Fn and hit the volume keys with one hand. And as we all know, changing the volume is a pretty frequent task.
And then I came across Function Flip (via Shawn Blanc).
Function Flip basically gives you the option of setting specific keys back to their default hardware functions. So I can use F1 through F9 as regular function keys and use F10 through F12 as hardware keys to change the volume without having to hold Fn.
It installs as a Preference Pane and getting up and running with it is ridiculously simple. It only does one thing, but it does it well and it's the one thing I need.
Good software should make you feel like you don't know how you lived without it. Function Flip is firmly in that category.