Install the latest SIMBL, and (you will have to uninstall the old one first by deleting the SIMBL folder from user/library/applicationsupport and user/library/inputmanagers) and then install the stable build of afloat. That should get it working on Leopard.
Best to check the Mac HD/library folders also for uninstalling the old SIMBL
A Leopard version was planned for January, but urgent matters (aka my
graduation) have preempted it; a tentative release date would be in
March, but I don't know if it's going to slip.
I've just stumbled across this topic, and adjei7's solution work.
I emailed the developer recently as well. He said the same thing as above; busy with graduation, but hopes to have it finished within the next few months. He said he didn't want to put a time to it, because it would most likely slip.
Well, I'm happy that I've got Beta 3 working with Leopard
Download Afloat 1.0 Stable Version Beta 3 (NOT Afloat 1.0b4)
Go to developer's site and you can still download the Afloat Stable Version Beta 3 on dmg. Open the dmg file, then use ctrl-click to Show Package Contents of the Afloat.PrePane file. Inside Contents>Resources, you'll find the Afloat.bundle. Drag that file out and drop into: