When speaking about Leopard only apps, Together is one I use most often. It's a real time saver and I really can't imagine doing my work without it.
Tip: Together uses QuickLook for displaying imported documents, so I suggest to visit
QLPlugins - Quick Look Plugins straight after installing Together. For example I use QLColorCode with Together to organize and tag my code snippets - works more than great!
Reinvented Software - Together 2.0 for Mac OS X - Keep Your Stuff Together, Find It Again Instantly