We support these external maps links and you may just shorten them directly:
http://maps.google.com/
http://maps.google.co.**
(country code)/
http://maps.yahoo.com/
http://maps.yahoo.co.**
(country code)/
http://www.mapquest.com/
http://goo.gl/maps/

http://www.bing.com/maps/
http://binged.it/
http://mapq.st/

If you think there are other external maps inks we should support with our service, please contact us.