[dwhly]

As the founder of a 501c3 non-profit that makes FOSS (http://hypothes.is), let me strongly second the points that Philip has made here.

1) Get expert counsel. It can make the difference in the ultimate determination, and also in the number of years (!) it takes to get approved. We were lucky in that we granted our determination without question-- and it still took nearly two years to get.

2) Don't focus on the software, focus on your mission. It's not just a ruse-- it really makes sense. Become knowledgeable about the nuances of the exemption, and why it is granted when it is.

3) Proactively call your agent, once one is assigned. Not only can it dramatically shorten the time to determination, but it can also often head off questions that, once asked, require formal replies and often lead to further clarifications, which all take time.