libcurl is a generic library for file transfers over a wide variety of protocols. Over the years, some of the recent ditributed version management softwares have learned about libcurl's powers and they now use it:
Anyone know of other version control systems using libcurl?
Ironies here include that libcurl itself is still kept within a CVS respository, and also quite possibly that the first version management project I myself participated is Subversion and that not only has two different HTTP dependencies, but none of those two are libcurl (they are neon and serf)...
Update: it seems that Mercurial is also using pycurl as an optional dependency.