I'm an intermediate java programmer. So far I've written a multi fractal generator (5500 LOC), an image processing library (2000 LOC), a graphical address book (800 LOC), a text adventure game and I have completed about 3/4 of the project euler challenges (the one's I understand). So I feel like I am pretty good at 2D graphics, GUI's, persistence, multi-threading and iterative mathematics.
I'd really like to take my skills a little bit further and take on a larger or more complicated project but I am fresh out of inspiration.
Have you got any idea's for an interesting project you would care to share?