We're having a discussion in the office about whether a button should become lighter or darker when a user hovers over it.
Here are some examples from the field:
Apple "Buy Now" button (Second is hover, third is depressed) - http://www.apple.com/iphone/
Twitter Bootstrap - http://twitter.github.com/bootstrap/
Github homepage - github.com
The button on FogBugz homepage goes from yellow to slightly lighter yellow. The buttons on Optimizely and Visual Website Optimizer hardly change.
Amazon's "Buy Now" button doesn't do anything when you hover over it (besides change to a pointer cursor). The colored buttons in Google's new interface (see Gmail or Calendar) go slightly darker when you hover over them.
Finally here is our button:
Should your button become lighter or darker when you hover over it? What else should you consider? Does anyone have data on whether the hover effect matters for conversions?