One of the things I love about our culture is InDay/HackDay. It works out to less than 20% time, but the entire company is given 1 day a month - generally the middle Friday - to do anything they want. This results in a lot of things ranging from prototyping ideas; learning new tools (I spent my day today brushing up on Scala); or just taking a fitness class.
The best part? The day is honored. It's not 100+n% time. Unless you own something that is bleeding money or users from a serious bug no one is going to come to you and ask you to do anything other maybe grab a beer.
I know we're not the only company doing this, I think Twitter has hack weeks every quarter, but it's a huge differentiator from our neighbors.