Simpler GUI for Jad, The Java Decompiler.
I naturally customized it to:
jad -o -s java -lnc -d%TEMP% %1
"C:\Program Files\TextPad 5\TextPad.exe" %TEMP%.\%~n1.java
Changed extension to java so that textpad does syntax highlighting; added -lnc so that jad outputs the original line numbers as comments and I can trace the exceptions I get
(after a few questions from people i'm editing this post:)
You will need to download jad and extract the exeutable to somewhere on your path, say C:\windows (or c:\winnt). Then create a new .cmd file, paste the above into the .cmd file, and put it somewhere. Then rightclick any .class file you can find, choose "Open With..."->"Choose Program"->"Browse..." and find the cmd file you created (don't forget to check the "Always use this program"). This way, any .class you'll double-click will decompile and open in textpad.