KOReader really is the big one; there are currently no Kindle hacks targeting the Scribe specifically, so basically nothing using the pen.
You can have a look at
NiLuJe's hacks in the Developer section for more hacks, but many of those are no longer relevant as they were developed a while back when the official firmware was very barebone. Much of the functionality those hacks provide (e.g. custom fonts, book cover as screensaver) are available in the standard firmwares nowadays.