Auto Awesome did an admirable—if unreliable—job of applying image adjustments to photographs uploaded to Google+, but clearly the idea here that a human touch may help somewhat more. So, there are now sliders for brightness, contrast, and saturation, plus highlight and midtone contrast controls. There are also local adjustment tools—which are really handy—and a range of different filters to play around with.
The features have been ported to work in the iOS and Android Google+ apps, but if you're at a desktop you'll need Chrome. It's also worth noting that the feature is rolling out slowly—so don't be alarmed if you don't see it just yet. [Google]