Android/Windows: PdaNet makes the software that makes iPhone-to-laptop tethering kind of easy. They've now released an Android app that's even easier to install, hook up, and use to get a net connection where Wi-Fi is scarce.
You'll need to connect your Android phone by USB cable—the developers say it officially supports the T-Mobile G1 right now—and launch the Windows software to both install and use PdaNet, but it's a lot easier than jailbreaking an iPhone. Simply run the installation executable, follow the instructions (pictured at right), and agree to install the drivers you're prompted about on your Windows system, despite their unverified/unsigned nature. You'll also need to turn on USB debugging in your Settings->Development screen.
Once you're installed, launching the phone-to-laptop tethering is a matter of launching PdaNet on your laptop, connecting your phone by USB, and then launching the PdaNet app installed on your phone. When your phone prompts you, right-click the system tray icon for PdaNet and hit "Connect," and you'll be up and running.
I tested PdaNet with my T-Mobile-provided G1, and it worked as advertised. PdaNet supports both 3G and EDGE connections, along with Wi-Fi for, we'd guess, older laptops or traveling desktops. Based on my browser speed while tethered with an EDGE connection, I'd really only use this for emergency web tasks that couldn't be knocked out on Android's browser or keypad, but 3G is likely more hospitable. I'd have provided screenshots of how the Android app's screen tracks data usage but, as you can see above, development kit screenshots and PdaNet don't play nice together. We'll note here that most cellular contracts either forbid or strongly discourage data tethering to computers over cellular connections, and the way that's found out is through excessive data usage, so you'd really want to make this a rare emergency/airport/internet-outage stand-in.
PdaNet is a free download for Windows systems and Android phones, and doesn't appear to have usage limits or time-outs. Thanks for the tip, Timothy!