I hadn't come across this useful set of collection helpers from Rob Rix before but I spotted him talking about some updates to it this week and had a look. Super easy mapping, folding and filtering of collections.
James Beith with a detailed look at working with dates and times in your iOS and OS X apps starting with the basics, going through parsing and formatting and ending up with the difference between YYYY and yyyy which you never know, might save some nasty bug one day.
Talking of dates, Anton Bukov has a lovely little UI component which takes the style of the iOS calendar view and gives it some features to be used as a picker control. Zoom out from days to months, then to years or a nice one week view.