I'm familiar with JAD, and haven't found anything newer or better, but I really didn't look that hard because I had previous experience with the JAD tool. However, I might have missed some really good options out there. Did I miss anything good? If so, what is it and what features does it have in comparison to JAD?
I've always used Jad, but there's also a decompiler called DJ that you may want to take a look at:
http://www.neshkov.com/dj.html
It has it's own GUI as well. Looks ok.
From Java 5 upwards there's also javap which dissassemble classfiles:
http://java.sun.com/j2se/1.5.0/docs/tooldocs/windows/javap.html