"Yahoo! Go is today’s essential solution for anyone with a mobile phone. It’s an all-in-one offering that lets you enjoy the best of the Internet on your phone - for free.* Send an email, upload photos, download a map, search for answers, check stock quotes, or get breaking news - all that and much more is available through Yahoo! Go."
A detailed list of supported devices is here, so see if yours is listed. Even if it isn't, it should work on Windows Mobile 5 and 6.