I'm setting up Raspberry Pi's using the Edimax EW-7811Us wifi module available on Amazon for a mere $11.

Following the Debian WiFi wiki page initially didn't work. The EW-7811Us uses an RTL8188CUS chipset which requires the rtl8192 kernel driver. There's no firmware-realtek package on Raspbian, and the best answer I found was to download some dude's hacked kernel module. No thanks.

Instead, install the rpi-update package then run rpi-update. The firmware will be updated in a way officially supported by raspbian (if there is such a thing). Then reboot.