Sometimes, you can learn a lot from just looking at a project rather than looking around for advice...
What are good state-of-the-art projects out there that are very good examples to learn from?
I had actually written a post on a very similar question - how do you read code ?
Most books, articles, etc on programming recommend taking a "good" open source project and "reading the code" as a way to improve your programming foo... Well, that's a good start but how do actually go about doing that is the problem. In this post I detail some pointers and approaches you can take - link.
Some recommendations (I work in .NET so are my recommendations) -
The question is a bit too generic, you should check in your field of interest. You can find here a good list of open source projects.
My personal favorite in term of code quality is Google Chrome (Chromium).
If you are a Windows developer and want to digg into the "Windows source code", you can check Wine or ReactOS.
For Java development, I recommend to check the Vuze BitTorrent client.
As a general rule, avoid huge and older projects because they have usually inconsistent code style and quality.