KINSole is a free, open source Win32 implementation of client side of the telnet protocol, described in detail in the RFC854 and related RFCs. It uses win32 console API and therefore may run both windowed and in fullscreen character mode. Unlike some other telnet clients it implements many telnet options, like "are you there", "timing mark", etc. described in RFC855, RFC857, RFC1091, RFC1073, RFC1572 and, probably, some others.

Screenshot of KINSole