How many extensions is too many? According to Natasha Murashev there's really no limit. I really like that this came originally from the desire for a #pragma mark replacement when Swift first arrived, but then turned into a better way to organise code.
Matthijs Hollemans works through the process of designing some types in Swift. It's a really well explained walk through of not only the solution, but the process of getting there.
Dominik Hauser with a look at his current thinking about the best way to architect a table view controller. I like posts like this because they take a concept we're all very familiar with and look at it with the new perspective that Swift gives us. I also like this solution because it's not over complicated.
Matt Gallagher digs into the weeds of Swift structs and reference counting. It's pretty obvious that you should never use some of the techniques in this article, but it makes an interesting read and you'll definitely learn more about the language.