Very popular free SSH client. Connects to hosts via secure shell, telnet or RS232. This program has been around for ages. It may not be the prettiest one, but it does the job and it is free.
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Pro De facto standard client for SSH, Telnet and Rlogin
Pro Source code available
Full source available. Compile and modify it yourself.
Con Lacks features
It is JUST an SSH client. There are many other options with built in X-servers, Multitabbing etc.
Con Tedious logging/tracing
Its tedious to set up logging and tracing (e.g. for serial connections).
Con Can not save passwords
Anyway, you can use "PuTTYgen" to generate a key pair, then use "Pageant" to do a password-less SSH remote login.
Con No global settings
If you want to change a setting for all your connections, you'd have to do it individually.