I'm currently developing an API for my site, and started writing the user documentation pretty early, which is something I haven't done before. The initial release of the API is only going to have a fraction of all the methods and resources I want to support, so writing the documentation like this has really helped me get my head around the whole system, how everything should work together and what will be needed in the future.

It's funny, because I always spend a fair bit of time writing notes when programming ... but it's usually a stream of consciousness straight to a notebook via pencil - sometimes with dialog as I ask and later answer questions to myself. This really helps me find solutions to problems I'm currently working on - but is not so good at later turning into user documentation. I've tried to use a word processor, thinking that it would be easier to manipulate later into docs, but it seems that the pencil, the paper and the process all contribute to the end product.

In this case though, writing the user documentation really helped me think about the overall picture, as not only was I trying to work out the best way to implement something, but also about how my users were going to use and learn it.