curl is a powerful library available on different platforms - I use it on Linux debian.
You can use it just to get http pages, or to post data and get a response from a server.
HTTPS is available too.
curl -k --data @request.xml https://127.0.0.1/service
posts the XML data contained by request.xml using a HTTPS connection (and ignoring invalid certificates, option '-k' - just for testing purposes).
This is a more interesting usage: it's nice to get info from Twitter or even send messages ("update your status") with just a command line:
Send a message:
curl -u user:password -d "status=describing how to use the Twitter API" http://twitter.com/statuses/update.xml
To install curl on debian:
apt-get install libcurl3
(install dependencies too, and I suggest to include libcurl3-gnutls-dev)