If you're like me and have an older (read "pre-2005") iBook or Powerbook and you miss the ability to enable the feature set that allows you to use 2 fingers on the trackpad to do various things, you'll definitely want to check out iScroll2.
iScroll2 enables all of the functionality that users of newer MacBooks and iBooks have on their trackpad. The specific feature that I was missing the most was the ability to put 2 fingers on the trackpad and click to get the equivalent of the ctrl+click combination that is normally required. Due to the fact that my Powerbook is running 10.3.9 it's hard for me to know if the ctrl+click functionality is something that is unique to 10.4, but I'm guessing that it is. I can enable the "Clicking" gesture in the Mouse & Keyboard control panel and that does enable the ctrl+click gesture previously described, but it also enables the ability to click by tapping the trackpad, which drives me crazy -- on Windows machines with trackpads, the first thing that I disable is tapping.
I don't really use and of the other two finger gesture shortcuts on my MacBook Pro, but I know people that do, so certainly check this program out if you need/want to enable that feature set on an older machine.
Note that this program is open-source, but does accept donations. Furthermore, rest assured that the installer package runs a check on your hardware to ensure that your hardware is compatible before running installation.