NetBeans with anti aliasing
I have Mustang as my primary VM for work and personal use. Yet I had to reinstall Tiger to be able to run NetBeans 5.0 beta2 properly because I kept getting a nasty exception regarding some XML things. Anyway, I couldn't use NB on Mustang, meaning I couldn't benefit from the subpixel anti aliasing. Hopefully, the NB guys told me today it is a bug in the JDK itself and it has been fixed in b61 so I installed a nightly build of Mustang b65 and it now works perfectly!
It looks so much better now! Unfortunately there is still a bug in Mustang's rasterizer which makes my favorite fixed width font (BitStream Vera Sans Mono free, GPL font) look really bad at small sizes.