[NiLuJe]

On FW 5.x, if you want active and free GUI interaction, you can either go the X way, with GTK (cf. what bad & twobob did, quite a few different stuff was ported), or the fb way (KOreader and the CR ports are pretty good example of that).

That imply C and/or C++ code and x-compiling (or slowly native compiling in a chroot, if you're twobob ;p), which many prefer to eschew, in favour of the more constrained but easier to approach other possibilities.

For example, there's also the WAF stuff for little web apps (cf. Komic, and the Native Menu).

And, indeed, the piggybacking on the screen-saver framework or just eips (which I guess is the tool you were thinking of, and has been shipped on Kindle FW for pretty much ever) to show static stuff, which can indeed be much faster and easier to achieve depending on your needs.

EDIT: And I completely forgot to mention Kindlets, which is the 'official' (java) framework for Kindle apps, which we can make a tiny bit more useful through the use of the Kindlet jailbreak. (cf. much of what ixtab has done, Collections Manager, KUAL, JBpatch, ...).