How to clean a rear derailleur? This has always bugged me. At the end of the season I'm breaking down my bikes and cleaning every inch of them and I get to the derailleur and can't seem to ever get all the junk out of the middle of it! Anyone have any good strategies for getting in by the springs and around the moving parts?

I'm fine with removing the cage and doing jockey wheels, etc., but would prefer to keep the main body together.
Lean the bike slightly against something with the derailer facing out.  Slide a section of newspaper up behind the derailer and large cog and let it drape down over the tire and onto the ground.  (This protects the tire and absorbs most of the solvent.)  

Get an old toothbrush and a spray can of (no shouting now!) WD40 or some other similar solvent.  Alternately spray the derailer (and cluster, if you're so inclined) with WD40 and work it with the toothbrush.  

When it's sufficiently clean (ie, no longer caked with congealed crud) wipe it dry and lubricate with chain oil.  (Since this is usually done in tandem with a chain cleaning, simply oiling the chain will usually oil the derailer sufficiently.)  

You may want to hit it with another shot of oil after a day or two, since the small amount of remaining solvent may prevent the oil from reaching all surfaces at first.