In the last few months of iPhone dev on apps such as HeyTell, we've learned a lot about how to save time and add features by taking advantage of third-party libraries. In no particular order, here are some of the easiest to integrate (thus "bolt-on"):
1. Reachability - Every iPhone developer should know about this. It's the Apple-approved way to check to see if network connectivity is available from your app. It helps to wrap this with your own code for generating helpful Alerts when the network bounces up and down.
2. SFHFKeychainUtils - The Keychain API may look scary, but it's useful too. How else will you store encrypted, backed-up key-value pairs that persist across uninstalls of your app? This wrapper API has just a handful of easy-to-understand methods.
3. Plug in an analytics API like the Medialets SDKs to capture information about your app at runtime. In just a few minutes you can be capturing app installs and start/stop events, a little more time gives you custom app events and lets you integrate ads into your app.
4. java-apns - Ashwin Phatak's Java library for push notification. Requires a bit of integration work (and a server component) but much better than rolling your own. (ok, so it's not a Cocoa Touch library per se - but it's essential if you're doing APNS in Java)
6. If you're a game developer you must have heard of OpenFeint, so there's no need to explain this popular social-media-enabling bolt-on library.
7. Appirater helps you get feedback on your app by requesting a review after a configurable amount of time or number of app runs. Just a few minutes to integrate.
8. The default HTTP client libraries sometimes don't cut it, especially when streaming or multi-threading. Try ASIHTTPRequest and get a wealth of features above and beyond what the standard NSURLRequest provides.
9. When building apps that parse XML from REST queries, you may want to try TouchXML which provides an easy-to-navigate DOM tree object from your XML documents. Don't like XML? Try Google's Protocol Buffers or Apache Thrift for more efficient and robust network protocol structures.
10. And when your project is ready to ship, give Hudson a try to automate your builds. The new Xcode 3.2 provides features that may help in archiving built products, but I think there's still value in doing a clean checkout and build from the repository (instructions for iPhone here)
We hope these give you some ideas on how to save precious mental CPU cycles on your next iPhone/iPod/iPad project!