Enough of a bitcoin client implemetation to do address calculations.
I wanted to generate some vanity pool addresses, but didn't want to trust the implementations of the address computation code that's out there. So I needed to implement enough ECDSA math to compute the curve points, and the standard bitcoin address serialization and deserialization functions to communicate the points to the vanity pool website.
There's not much more of the implementation than that. I started writing a MtGox API, then MtGox collapsed and I kind of lost intereset for a bit.
I was learning F# at the time, and found it to be a great language for this task.