libcurl - the multiprotocol file transfer library
libcurl is a free and easy-to-use client-side URL transfer library, supporting DICT, FILE, FTP, FTPS, Gopher, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, POP3, POP3S, RTMP, RTSP, SCP, SFTP, SMTP, SMTPS, Telnet and TFTP. libcurl supports SSL certificates, HTTP POST, HTTP PUT, FTP uploading, HTTP form based upload, proxies, cookies, user+password authentication (Basic, Digest, NTLM, Negotiate, Kerberos), file transfer resume, http proxy tunneling and more!
libcurl is highly portable, it builds and works identically on numerous platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HPUX, IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, BeOs, Mac OS X, Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS and more...
Go to the regular curl download page and get the latest curl package, or one of the specific libcurl packages listed.
You use libcurl with the provided C API. The curl team works hard to keep the API and ABI stable. If you prefer using libcurl from your other favorite language, chances are there's already a binding written for it.
libcurl is most probably the most portable, most powerful and most often used C-based multi-platform file transfer library on this planet - be it open source or commercial.