My law firm uses Helpself, which we've found to be incredibly user-friendly (like 5-10 minutes and you can create a document assembly flow). It's www.helpselflegal.com
I know they can also build out custom document assembly platforms if you want something really complex that you have design requirements for.
We use a software called Documate which is built on the docassemble platform. I think it's the same as the one a user mentioned below (HelpSelf). I LOVE it. We use it to use the intake process at our law firm to populate all the documents in a case. We also have a whole automated forms library set up on Documate, which makes it easy to populate lengthy contracts quickly. We've also tried other document assembly and document automation tools like HotDocs and Contract Express, but they were incredibly expensive and hard to figure out how to use.