Yeah I think it's called PiFast.
There's a few others I tried too, pi_css5, qpi29, simplepi, etc. PiFast was the best. Not all of the programs I tried allowed me to calculate 1 billion digits though. After all, it is a big number!
If you're gonna try to calculate it it's probably best to have a lot of memory. I've never seen my page file so big.
I'm gonna try to write my own pi calculator.
Oh yeah, the output file was a 1.3GB .txt file, and I couldn't open it in one piece so I also used something else to split the file into a couple of hundred smaller pieces. That was called "jsplit" or something.