I found this list of OpenSim grids built for the Second Life ™ client.
The last time I looked there were only 2 and now there are over 20 – more “grids” and “standalone sims” than Second Life ™ started with in 2003. Simultaneous with the rise of these Linden Lab has slashed private island prices and made low-prim sims even more affordable. It seems like only a matter of time before one or more of them support a micropayment function.
Check out the list at: