libssh2 and libssh both provide an API to develop SSH based applications.

Here's an attempt to put some light on the differences between them.

• License: 3-clause BSD License
• Developped in: C (30218 SLOC), sh (1102 SLOC), Perl (65 SLOC), Lisp (33 SLOC), AWK (23 SLOC)
• NUmber of functions: 170
• Key Exchange Methods: diffie-hellman-group1-sha1, diffie-hellman-group14-sha1, diffie-hellman-group-exchange-sha1, diffie-hellman-group-exchange-sha256
• Hostkey Types: ssh-rsa, ssh-dss
• Ciphers: aes256-ctr, aes192-ctr, aes128-ctr, aes256-cbc (rijndael-cbc@lysator.liu.se), aes192-cbc, aes128-cbc, 3des-cbc, blowfish-cbc, cast128-cbc, arcfour, arcfour128, none
• Compression Schemes: zlib, zlib@openssh.com, none
• MAC hashes: hmac-sha2-256, hmac-sha2-512, hmac-sha1, hmac-sha1-96, hmac-md5, hmac-md5-96, hmac-ripemd160 (hmac-ripemd160@openssh.com), none
• Authentication: none, password, public-key, hostbased, keyboard-interactive
• Channels: shell, exec (incl. SCP wrapper), direct-tcpip, subsystem
• Global Requests: tcpip-forward
• Channel Requests: x11, pty, exit-signal, keepalive@openssh.com
• Subsystems: sftp(version 3), publickey(version 2)
• SFTP: statvfs@openssh.com, fstatvfs@openssh.com
• Thread-safe: just don't share handles simultaneously
• Non-blocking: it can be used both blocking and non-blocking
• Your sockets: the app hands over the socket, calls select() etc.
• OpenSSL, Libgcrypt or WinCNG (native since Windows Vista): builds with either

• License: GNU Lesser General Public License
• Developped in: C (46021 SLOC), C++ (1181 SLOC), sh (186 SLOC), Python (9 SLOC)
• NUmber of functions: 213
• Key Exchange Methods: curve25519-sha256@libssh.org, ecdh-sha2-nistp256, diffie-hellman-group1-sha1, diffie-hellman-group14-sha1
• Hostkey Types: ssh-ed25519, ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521, ssh-rsa, ssh-dss, ssh-rsa, ssh-dss
• Ciphers: aes256-ctr, aes192-ctr, aes128-ctr, aes256-cbc (rijndael-cbc@lysator.liu.se), aes192-cbc, aes128-cbc, 3des-cbc, blowfish-cbc, none
• Compression Schemes: zlib, zlib@openssh.com, none
• MAC hashes: hmac-sha1, none
• Authentication: none, password, public-key, hostbased, keyboard-interactive, gssapi-with-mic
• Channels: shell, exec (incl. SCP wrapper), direct-tcpip, subsystem, auth-agent-req@openssh.com
• Global Requests: tcpip-forward, forwarded-tcpip
• Channel Requests: x11, pty, exit-status, signal, exit-signal, keepalive@openssh.com, auth-agent-req@openssh.com
• Subsystems: sftp(version 3), OpenSSH Extensions
• SFTP: statvfs@openssh.com, fstatvfs@openssh.com
• Thread-safe: Just don't share sessions
• Non-blocking: it can be used both blocking and non-blocking
• Your sockets: the app hands over the socket, or uses libssh sockets
• OpenSSL or gcrypt: builds with either
• Client and server support
• SSHv2 and SSHv1 protocol support
• Supports Linux, UNIX, BSD, Solaris, OS/2 and Windows
• Automated test cases with nightly tests
• Event model based on poll(2), or a poll(2)-emulation.

The same data, in a table: