From nikf at the Brighton iPhone Developer Group. Will have to test these out when I get a chance, as I do use quite a bit of HTTP (and XML) code in the app Im working on.

• Fade Out Your App’s Default.png file - Totally self-explanatory, with code for your use, it’s a nice touch to add to any app.
• Speed Limit and SlowMo - If you’re wanting to recreate cellular network bandwidth and device-like resources, these two are super-handy. Though, don’t negate the need for testing with a crappy real-life cellular network. (Thanks to Aral for reminding me about SlowMo)
• HTTPClient - Great for testing and debugging any HTTP requests, this saves me so much time when working with a particular web API for my app (if you’re curious, it’s not the Twitter API).
• iPhone Backup Extractor - if you want to see what your application is saving to the device, this app is unbelievable handy at extracting user data and preferences from iPhone backups.
• Programming the iPhone User Experience - This isn’t a complete in-depth reference for your perusal, but it’s a great reminder of things to bear in mind when building an app. It’s also a decent enough size to keep in a slimline rucksack (as I am right now).

Update: Seasoned developers might also want to take a look at AQXMLParser. It’s awesome.