GMGridView - we needed to build a large scale grid view with dequeueable cells, so we gave GMGridView a try. Wow. It feels like something that Apple should have built. Well defined delegates, great gestures (including drag and drop reordering of cells, and pinch and zoom cells to transition to a more detailed view), different layout strategies, and much more.
SVProgressHUD- this is our standard pop-up notification in place of UIAlertView. Simple status, error, and dismiss methods make this dead simple.
SORelativeDateTransformer - no one wants to see dates these days. Everyone wants relative dates like '2 minutes ago' or '1 week ago.' This single class library provides the desired functionality. As a bonus, it is in English, French, Spanish, and German.
RLQueue - this is an NSOperationQueue replacement that has processing, success, and error blocks, as well as setting priority and allowing the cancelling of an operation.
GTMNSString+HTML - this is just a single class out of the Google Toolbox for Mac. It converts HTML encoded characters to their actual character. For example, it will convert " to ".
SCYahooWeather - we needed a simple weather API to convert lat/long to current weather conditions. I prefer to use a documented API rather than an undocumented one (e.g., Google Weather). This library simplifies the Yahoo Weather API, although you do need to make two API calls, the first to convert lat/long to woeid and the second to use the woeid to retrieve the current weather. A little roundabout, but it was sufficient for our use case.
TTTAttributedLabel - for one project we had the need for a label that allowed us to change fonts within sections of the label. TTTAttributedLabel is a nice wrapper around Core Text that allows for this. It also allows for links within the label.