The latest stable release of libsurl is libsurl 0.7.1.
Source downloads of libsurl are available through the project download page on launchpad. Likewise, source downloads of the surl command-line tool are available on its own download page. Binary builds are not yet available, although ubuntu packages will be made soon.
Both libsurl and surl are believed to work on all unix-based systems. So far, there is no windows port available for libsurl, and surl requires libsurl to run, so neither can be run on windows (except under an emuation environment such as cygwin).