Sapien make some great tools and this is no exception. PrimalForms Community Edition is a Free version that has limited functionality.
I've used this for several OSS projects and it covers the essentials. The link on Sapien sites no longer works so below is a alternative link I've found. This version is 18.104.22.168 but an version 1.0.10 was released also according to the blog. Only obvious change is it adds a start page.
Update: Unfortunately Sapien or one of their representatives has mistakenly slapped DMCA on this link for the free edition and I've removed it, rather than fight it. You can certainly find it via Google or ask the right person ;-)
Signed By: Sapien Technologies, Inc.
File size: 4,689,920 bytes
Version 22.214.171.124 was released, but I'm yet to find a copy of it.