Extend the Blog GTK program to be more like Windows Live Writer or Blogo (neither is open source).
Extend ePub tools to support HTML to ePub (open ebook format).
How about a native version of F-Spot that is more like Picasa?
Same for DocBook or TEI.
Gimp "fashion" makeover; make Gimp more like Photoshop. This probably means redoing it in GTK or Qt. A good example of a project that recently did something along these lines is "emacs-snapshot".

Web and Web Services Applications

Cloud-enabled LaTeX editing. (non-cloud version will do though) Basically, can we build a version of LyX for the cloud? The eqn editor was built as part of Google Summer of Code.
Please give me the equivalent of restlet.org for Python! Maybe Pylons is the answer.wsUnit testing framework for web services?

Systems

Contribute code to the Linux kernel (e.g. develop a proper driver for the new Elantech touchpad mouse with desktop integration for making proper mouse and touch settings)

Languages

Port jsonselect to Python (or whatever other language you would like).

Port a project to Python 3 (find projects in need of porting on the Python 3 Wall of Shame)

Mobile and Embedded Development

Being Helpful to Existing Projects
Fixing Bugs. Example: Fix bugs in the Linux kernel or in Python (or anything). Show evidence of your participation in the form of forum discussions and commit traffic.
Documentation: Write user documentation for a project that would make people want to use it. (e.g. how to write a Pylons program.)