There's a few, but I think it's still too early to really call out a "defacto" framework.
Revel, as mentioned, is a popular "full stack" framework, but it can be pretty heavy. web.go[1] gets a lot of use (it sits at a Flask/Sinatra level), and beego[5] looks good but it's also totally acceptable to plug a few libraries together without much work on top.
For example, I'm using:
- mux, sessions, schema & context from Gorilla[2]
- go.auth for social sign-in [3]
- the RethinkDB driver [4]
... and that's about it. Write your routes with mux, set up your handlers with auth/sessions/schema, and generate your models & queries with whatever DB you want. The standard html/template library is good enough (and there's a Mustache library if you need it), and the standard http library covers everything in between.
TL;DR: If you enjoy writing Flask/Sinatra apps, you'll enjoy writing Go web apps. If you're coming from a full-stack framework (like me), there is definitely a learning curve, but I feel that Go's documentation and std. lib flatten that out a little.
[1]: https://github.com/hoisie/web
[2]: http://www.gorillatoolkit.org/
[3]: https://github.com/bradrydzewski/go.auth