OpenSSH/Other SSH Implementations< OpenSSH
Dropbear is a smaller, modular, open source SSH2 client and server available for all regular POSIX platforms. Dropbear is partially a derivative of OpenSSH. It is often used in embedded systems because very small binaries can be produced. Functions that are not needed can be left out of the binary, leaving a lean executable. Thus a working server can be boiled down to 110KB by trimming away various functions.
Many distributions and products use Dropbear. This includes OpenWRT, gumstix, Tomato Firmware, PSPSSH, DSLinux, Meego, OpenMoko, Ångström (for Zaurus), ttylinux, Sisela, Trinux, SliTaz, Netcomm, US Robotics, some Motorola phones, and many, many more.
Tectia is from SSH Communications Security Corporation which is based in Finland. It is a closed-source SSH client and server with FIPS support.
Solaris Secure Shell (SunSSH)Edit
Sun SSH is fork of OpenSSH 2.3, with many subsequent changes.