Launchd Services makes it easy to add startup tasks to OS X. Go4Launch does all the behind-the-scenes work and creates a "template" shell script that you can modify (e.g., sudo vi /Library/BootScripts/go4launch.sh). You can add your own commands to this shell script and save it. Lingon is a better choice if you want to manage launchd services. This app is simple and easy to use, making it ideal for DIYers.
Version 1.2: