Internet Explorer: 2,083 characters, with no more than 2,048 characters in the path portion of the URL.
Firefox: After 65,536 characters, the location bar no longer displays the URL.
Safari: At least 80,000 characters will work.
Opera: At least 190,000 characters will work.
RFC 2616, "Hypertext Transfer Protocol -- HTTP/1.1," does not specify any requirement for URL length.