Wanna sideload apps (or use ADB for any other reason) to your Kindle Fire via your computer? It's really easy -- just add the USB Vendor ID to the Windows ini file. Even reading it seems more complicated than it really is. On your Windows computer, press the Start button, then choose "All programs" > "Accessories" > "Command prompt" to open the text based command prompt for your computer. Then copy the following line, and past it into the command window:
Press the Enter key, and you're done. What's it doing you ask? It's adding the Vendor ID (0x1949) to the end of the Users\%your_user_name%\.android\adb_usb.ini file, so that the Android SDK tools can communicate with your Kindle Fire. Doing it via the command prompt means you don't have to make hidden and protected files viewable, and it has a little bit of nerd factor to make you smile inside.
Once you've done that, enable Unknown sources in your Fire, connect it to your computer, and you can install apps just like you would with any other Android device. For more help on installing and setting up the Android SDK, visit the forums.