You put it on github under an MIT or BSD or similar license. So corporations or anyone else can download and install your software and use it at no cost.
On your main website, you offer the software but on a per-user fully hosted, running online basis. So your revenue will come from people signing up to use your fully hosted service where they don't need to make any effort to install or configure - it just works. You also make money offering support contracts to people who have downloaded and installed their own copies of the software.
The reason you have open sourced it is to spread the word, gather interest and build community around your product. The theory being that the vast number of free users will be the impetus that leads to a small number of paying people discovering and using your product. Maybe free and open source is more newsworthy, more likely to get press and blog coverage?
Theoretically anyone could take your code and set up in competition to you. Anyone could fork your code and rename it to something else.
So is open sourcing your SAAS application a good way to do business, or is it foolish and giving away the farm?
Is there any examples out there of companies that have actually made money taking this approach?
I'm scared that if I fully open source then I will somehow have given the value away.