In my daily work I always needs to have remote access, either telnet, ssh or rdp to Unix or Windows based machines. I have used various open source remote access clients and here are them.
Among the open source remote access software, putty is definitely one of my favorites. Using it together with Putty Connection Manager, you can further customize it for a tabbed based interface. Another choice is to use WinTabber to achieve the same thing.
Kitty is a fork from putty with many features added.
Another good remote access software is Poderosa. It also supports a tabbed based GUI natively.
mRemote is another full features, multi tabbed connection managers. Besides telnet/ssh, it also supports RDP.
TeraTerm is another one that I used before, but looks like it is not being updated for long time.
WinSCP is the one I used for SFTP and SCP access.
Terminals is a secure, multi tab terminal services/remote desktop client. It uses Terminal Services ActiveX Client (mstscax.dll).
dtelnet is another free telnet client for Windows