My understanding is that Skylark made it first around 2009 after difficulties running Powder Game in Linux. He used to host it on his website,unaligned.org. He got bored with it/had to abandon it for other projects and shut down the powder server. Then Simon took over and created a new TPT server,the one you're using now.

I'm pretty sure one of cate's comics was about the origins of TPT,but I don't remember.

If you want to see TPT in its very beginnings (read: a good number of penis saves and unmoderated cancer) ,you can stop by http://powdertoythings.co.uk/OldPowder/ .