Apply Shape Keys and Cloth Sims

So, you've created a curtain with the cloth simulation and now you want to turn it into a regular polygonal mesh.

Pick the frame of the simulation you like best.
Create a New Shape Key From Mix.
Delete the old Shape Keys and then the Mix.
Press Apply on the Cloth Modifier.

And there you go, a regular old-fashioned polygonal mesh, just like your grandmother used to make.