I'm using XMaster and I'm very happy with it. I've only had a few crashes, mainly when disabling hacks, and those were all recoverable with a soft reset.
My favorites are EasyLaunch, which lets you assign applications to pen strokes and button presses, and MiddleCapsHack, which lets you write capital letters more easily.
I also use PalmWiki to link ToDos to existing memos (e.g. a project or reference memo) and I'm evaluating Hi-Launcher, a start-menu-like application launcher.