MultiFireFox 2.0
Given the fact that lots of folks are downloading FireFox 3 as of late, I thought this might be a good time to highlight MultiFireFox 2.0. MultiFireFox 2.0 lets you run up to 5 versions of FireFox side by side.
Why would you want to do this? If you're a web developer you probably want to check your work in FireFox 2 as well as 3. MultiFireFox also lets those of you who don't want to jump into the new version of FireFox get your feet wet with the new hotness (or should that be the 'new wetness?'), but keep FF2 close at hand.
MultiFireFox 2 is free, so you can't beat the price and runs on OS X 10.4 and above.
