I've been playing this evening with making a version of Weathermap that can draw maps using the Cairo graphics library, instead of GD, which it presently uses.
Cairo is the library that the next version of rrdtool uses, along with other large projects. It offers some interesting features like proper alpha handling/anti-aliasing and more output options (PDF, postscript, as well as the usual PNG etc).
Here's a side-by-side comparison of GD and Cairo on the same map - my "0.96 testing" map. I didn't actually rewrite Weathermap to do this - I just made it write out the coordinates as it drew things, and wrote a perl script to read those and use Cairo to draw them again. That means not everything is in the Cairo image, but there's enough to see the differences.
Here's the complete GD image, and here's the Cairo one. The anti-aliasing is the most obvious difference, on the curves and curve outlines.
Only downside is that there aren't really any good php bindings for Cairo yet.
The GD map shows a few new 0.96 features too, including pie and nink artificial icons, and z-ordering (half the double link goes below the big box, and half goes above - same for the items around the legend).