These are all mine changes I made to YUI compressor in a past time:
-replace this as var if occurs more than once in that scope
-one var per scope. all variables are declared at the beginning of scope.
-hint for eval statement (so YUI can know if eval creates/uses local vars) and so can safely obfuscate scope with eval
Example:
var x = 1;
"use:x,create:z";
eval("y=x+1;var z=2;");

-cut leading zero for decimal numbers (-0.1 becomes -.1)
-fixed breaking line in for loop (for loop must be in one line)

I'm using all this code in my small private projects but it needs some heavy testing before it's pushed to release.