I received an otherwise uninteresting spam email today, with an interesting link:
http://rds.yahoo.com/S=1933683/K=computer/v=3/SID=y/l=WS1/R=1/SS=49221344/IPC=us/SHE=0/H=0/SIG=91234gwS08/EXP=564103787/*-http://google.com.mtg****.net/home.asp (I have ****'ed out the final domain name to avoid giving them any additional advertising).
The above is a fully valid url - ending up at Yahoo!. Yahoo then is kind enough to redirect the unfortunate user to the spammer's site: http://google.com.mtg****.net/home.asp
As best I can tell, Yahoo uses this service to redirect (and track) clicks on ads, etc. They have another service that does the same: http://rd.yahoo.com, but at least it's smart enough to check the final destination: http://rd.yahoo.com/*-http://google.com (or maybe it just uses a different scheme).