At our last Melbourne Force.com Developer User Group meet up, I did a short presentation on some of the features in the Summer '13 but specifically focused on developers, the idea being to highlight some things that might have been looked over since the majority of release overviews tends to focus on new product features.
Rather than write up the slides here I've thrown together (somewhat hastily, it could definitely do with more polish) a video of the presentation I did, squished down into a little over three minutes which you can watch below.
Given my recent spate of looking at test methods, items of particular interest to me have been the changes to test methods. Not only do we have a very handy new annotation, @TestVisible, but now test methods must be written inside of test classes and can no longer be placed in the same class as other code (if you're saving using API 28.0 onwards). There's some great reasons for this though rather than explain them myself I highly recommend reading the blog post written by Josh Kaplan on the subject.
Summer '13 for Developers
My First Pluralsight Training Course: Introduction To Visualforce
Aside from the video above, I'm also pleased to say that my first training course for Pluralsight went live this week. It's called Introduction to Visualforce, and covers a large amount of the standard functionality available, including features of Standard Controllers Standard List Controller. If you're just getting started on the platform (or even if you've been using it a while), it's vital stuff to know: I implemented pagination more than a couple of times before I found out it was already supported. Please check it out and let me know what you think.