Stack Overflow posts are CC-licensed and all the data is available at https://archive.org/details/stackexchange - this is where Zest downloads it from.

Dash also provides SO search, though only in titles (no full text search).

I think this is going to have a different target audience. Dash includes tons of documentation from very diverse set of sources, including .NET and Apple docs. Zest supports only DevDocs and Stack Overflow. It is also open source and works on more platforms, so may be eventually better for people preferring FOSS.

Zest is also pretty much an experimental proof of concept at this point and thus far from being as polished as Dash.

However, I think full text search is a very cool feature which Dash is currently missing, which was the main factor which motivated me to go ahead and try implementing it.