For personal projects, working independently, I'd used a couple of Objective-C open source libraries. SBJSON  converts JSON text responses from a web API to NSDictionary objects, and MBProgressHUD  provides a loading animation that can be used to improve the user experience during longer running processes. I had the opportunity to do some development on the primary consumer iOS app where I work (a top 10 free "lifestyle" category app), and it uses a lot more open source projects than the 2 mentioned above. The list below contains some of those libraries, in use by the application (iOS version 4 or earlier as of this writing).
Unfortunately I don't have direct experience with all of them, but I wanted to share them as a starting point for further investigation by myself and others, and to provide a little extra advertising for the projects.
TTStyle and TTUrlRequest - Part of Three20  from Facebook. ttstyle provides various UIView features like borders, fills, text and more. I typically used Interface Builder to lay views out, but have seen the flexibility and control of building views programmatically, and now prefer that approach. tturlrequest is an HTTP request library, from the creator: "a replacement for NSURLRequest which supports a disk cache." A cached web request response on disk would be available between application restarts.
FTUtils  - Animation library. Ships with some canned UIView animations. I have not used this personally.
GTM  - Google Toolbox for Mac. Various features. Has OAuth connection controller code for iOS.
oauthconsumer  - OAuth consumer library. I have not used this.
regexkitlite  - Adds additional regular expression functionality beyond what objective-c provides, to NSArray and many more objects, through categories.
KIF  - User interface-level (integration/"outside-in") testing from Square. I have not built a KIF test script yet, but testing a happy path flow through the application would be a valuable regression test.
Google Analytics SDK for iOS  - Track activity in google analytics, "pageviews", events, and more.