[universa1]

And there's a new one in the making, hopefully sticking to the more classical settlers 2-4 mechanics and feel... And not the later iterations of the series...

There is also Foundation, by a smallish canadian studio (iirc), that kinda is a mix between settlers and sim city :-)

[kzrdude]

There is an open source reimplementation of the settlers II engine, called RttR and it works flawlessly (better than the original for me).

There's also an open source clone of Settlers II called Widelands.