Update 2020: I have a new answer that replaces this, now 8-years-old, one: https://stackoverflow.com/a/61177330/278976
On Linux, I tried cpp-netlib, libcurl, curlpp, urdl, boost::asio and considered Qt (but turned it down based on the license). All of these were either incomplete for this use, had sloppy interfaces, had poor documentation, were unmaintained or didn't support https.
Then, at the suggestion of https://stackoverflow.com/a/1012577/278976, I tried POCO. Wow, I wish I had seen this years ago. Here's an example of making an HTTP GET request with POCO:
https://stackoverflow.com/a/26026828/2817595
POCO is free, open source (boost license). And no, I don't have any affiliation with the company; I just really like their interfaces. Great job guys (and gals).
https://pocoproject.org/download.html
Hope this helps someone... it took me three days to try all of these libraries out.