I have a jailbroken iPhone, and I wanted to add an Activator Event for a wrong passcode, so that entering '0000' would send a text message. I looked through a bunch of tweaks without any success, so I decided to make it myself. I jumped into learning the basics of Objective C, and through the iPhoneDevWiki and open source tweaks on GitHub, I finally managed to make PasscodeActivator. The source code is up on my GitHub if you want to check it out.
If you want to install it, simply add my repo, http://cydia.alexbeals.com, and install PasscodeActivator.
It was challenging trying to pick up how to do it because very few jailbreaking development tools are documented, and a lot of the work is done by looking through header files (this was a great resource) and trying random functions based off of their names. 'CrashReporter' and 'syslogd to /var/log/syslog' were two essential tweaks in developing, in conjunction with a LOT of NSLog statements. Overall, it was a good project to start with, as it incorporated using external libraries, preferences panes, notifications, and saving to files, which gave me a rough overview of a lot of areas. The best tweaks to develop are ones that you will use, so I think my next project will be working on an overhaul of the Emoji keyboard.