The ready-made marketplace is in interesting point. When I was using a Zaurus, I frequented handango.com and was able to find some good, commercial apps that I gladly purchased. There are currently two apps that I would pay good money ($100+ USD): Something like Textmaker and a good, flexible database program.
At one point I actually contacted Softmaker to see if they would be willing to port Textmaker to the N800, and the answer was no - albeit understandable, considering their other mobile linux offering was based on QT3.