libssh2 and libssh both implement SSH and provide a library API for apps. Both support SSH, SFTP, auth, channels etc. Both are 25K-30K lines of code.

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

• BSD licensed
• Can set the API to either blocking or non-blocking
• client-side only
• stable API
• strict name space - everything is in libssh2_* something
• C89 compatible
• SSH v2 only (not v1)
• all 100+ functions are documented in their own man pages
• autotools-based building

• LGPL licensed
• Has a blocking API, although parts can be used non-blocking
• client-side and server-side support
• API that still changes
• name space pollution - uses lots of (generic) prefixes like 'ssh', 'channel', 'buffer', 'string' (and some even without prefix)
• C99 support required
• optional SSH version 1 support
• doxygen-generated docs
• CMake-based building

(It could be noted that libssh's primary development is nowadays done in their 0.5 branch and once that becomes their release branch we'll update this comparison)

The same data, in a table: