How to Sideload Android Apps

1. Get apk files
2. Transfer those .apk files from your PC to your Android however you like: on a microSD card through a cloud service such as Dropbox, via a USB transfer, etc.
3. Tweak Application settings (default setting in Android is to not allow the installation of non-Market applications)
4. Go to your Android device's Settings, and under Applications, check the "Unknown sources" box allowing you, in effect, to sideload.
5. To install your .apk apps, you essentially launch the files through a file manager app. Many Android devices come with a file manager preloaded. Good onees are Metago's Astro File Manager or Rhythm Software's File Manager HD for Honeycomb tablets. For folks locked into the Amazon Appstore (users of the Grid10, Kindle Fire, etc.),  ES File Explorer is good
6. you just need to launch your file manager and find them
7. Touching an .apk icon will open the Package Installer, where you can touch the Install button to finish the deed

The Kindle Fire is easily accessible via the mini USB connector. I created a directory called Temp and dump my apps to sideload into that. Once the app is copied onto the Fire, I open Temp with the Astro File Manager and select the app. It will ask if you want to install them.
Some of the HD apps that work with the Fire include:

• Pinball HD for Tegra
• Air Attack HD
• Majesty
• Star Traders