Still waiting for something like Express in Go but I doubt it'll happen. I don't think anyone is serious about web development with Go based on what I see in the IRC channel and newsgroup on a daily basis, the lack of libs doesn't help either.
Edit: I want to add that I attempted to step up and learn Go as I made a web lib similar to Express. I had a little success but most of the time the people on the IRC channel weren't very helpful.
Most people didn't understand the need for such a lib (something like Express), it's just a bunch of people parroting "it's so easy in Go, you don't need a lib!" but these are people who never built a single web app in their entire life or going into attack mode saying anyone who needs a super light weight lib is some type of scrub developer.
It made me instantly turn my shoulder to the Go community and it'll take some pretty good libs to make me change my mind because in the end getting stuff done is the only thing that matters and Go + reasonable web dev isn't close to solving that problem yet.
I wasn't sure how to decouple the middleware functionality out of your lib so it could be applied to a different router.
Web.go (875 stars) https://github.com/hoisie/web
Revel (742 stars) https://github.com/robfig/revel
Beego (264 stars) https://github.com/astaxie/beego
Falcore (224 stars) https://github.com/ngmoco/falcore
Mango (160 stars) https://github.com/paulbellamy/mango
Golanger (159 stars) https://github.com/golangers/framework
if you need simple routing and REST url parameters this is a great library to use. I've found it outperforms many of the other routing libraries. The best part is that it (mostly) uses the standard net.http library and you don't need to code against custom APIs.
there is also Gorilla Web Toolkit: http://www.gorillatoolkit.org/
this library has been around for a couple years, and is often recommended on the Go mailing list. It recently switched from Google Code to Github, so it doesn't have as many stars as the others. It provides routing, secure cookies, sessions, and more.
I personally would start with Gorilla or Pat, because they are very thin layers on top of the standard net.http library with little abstraction ... in many cases the standard library works great. If you end up needing a higher level framework you can always switch to something like Revel.