While CAPA and PLA are the primary materials, there are also experiments with ABS, at:
http://hydraraptor.blogspot.com/2008/04/absolution.html

and HDPE, at:
http://hydraraptor.blogspot.com/2008/03/hdpe-pu.html

At this point the extruder can extrude any thermoplastic with a melting point of less than 200 Celcius. A high temperature extruder made of stainless steel is being worked on, and if it works it would be able to extrude the full range of thermoplastics. That experiment is at:
http://hydraraptor.blogspot.com/2008/03/high-temperature-extruder.html

There is work on high resolution printing, like at:
http://forums.reprap.org/read.php?1,10726

The priority, however, is making something that can make the majority of itself. Because then you can make the custom parts needed for high resolution printing cheaply; for a typical material cost of twenty dollars, rather than having to order several hundred dollars of parts just for an experiment.

For fun stuff that can be built with low resolution plastic, hopefully I'll be able to print a small kayak next summer:)