MTPuTTY (Multi-Tabbed PuTTY)
Written by Simone
Nice little piece of software; free but not open source.
Use your saved PuTTY configs in a multi tabbed window! It's just easy like that, no fuss, no complicated setup but with added options, like saving your sessions when quitting the program, so that they'll run again at next startup; auto reconnection on connection loss, with a timer, a few themes for both light and dark setups and possibility to run scripts in any or all of your SSH sessions.
If I'd have to be picky, there's one thing which doesn't work: flashing window on bell, but maybe that's expected since it's just the main window with different tabs.. Well, I think it could be improved anyway.