Mac only: The iPhone 2.0 jailbreak utility for Mac, PwnageTool, has just been updated to accommodate the latest iPhone software update from Apple. Re-jailbreak your iPhone 3G, first gen iPhone, or iPod touch running the 2.0.1 software with PwnageTool 2.0.2. PwnageTool creates an alternate firmware bundle to install on your device, that includes an app called Cydia—and new to this version of PwnageTool—a new beta of Installer.app. Those two apps serve as gateways to software repositories you're not going to see in iTunes' App Store. Here's a screenshot of just a few apps I installed via Cydia and the Installer.app post-jailbreak.
On the top left you see Installer and Cydia, then gTxtEdit (a better text editor than Notes), ModPlayer (plays more file formats than your iPod), SynchStep (plays songs with beats that synch to how fast you're walking—seriously), MxTube (a YouTube clip downloader), and the NES emulator.
If you install OpenSSH from Cydia, you can also copy media from it to your computer and tether your iPhone to your Mac to get online anywhere.
Here's the step-by-step for jailbreaking your device with PwnageTool; developers recommend upgrading via the original PwnageTool's built-in update system. PwnageTool 2.0.2 is a free download for Mac only.