Being a linux based mobile device user, I only have 2 applications that I purchased.
One of them is an audio player that I first purchased because it was the only product that played ogg files and had a GUI. The application is great and I would happily buy more software from the company - except the support from the company (and the president in particular) for bugs and requests is abysmal.
The second application is a Japanese input method, Effy from
http://www.timespace21.com/eng/, I got a discount for helping beat test the app for Zaurus, but after using the app I would have bought it anyway. Their support was excellent and app is too.
The rest of the apps that I run on a regular basis are all GPL apps and therefore are available for free (usually in both Free Beer and Free Speech meanings of "free"). Several of the programmers have ways to make donations, but some of them just do it for the love
Most of the open-source software that I've used on the Zaurus are better than the software that Sharp packaged with the device and any commercial versions available.
Stu