Per-directory customizations?

[Roy Smith]

Is there any way to do customizations on a per-directory basis?

For example, I've got java project which lives in ~/dev/foo.  Anytime I 
run M-X compile anywhere inside the ~/dev/foo hierarchy, I want the 
command run to be "cd ~/dev/foo; ant".  I don't want to just set 
compile-command in my .emacs file, because the right command varies 
depending on which project (i.e. directory subtree) I'm in.

Is there any way to do this?  I'm running GNU Emacs 21.2.1.