lsusb listed my adapter as
USB: 050D:705A Belkin Components F5D7050A Wireless Adapter
Then add a "non-free" component to /etc/apt/sources.list for your Debian version.
deb http://ftp.us.debian.org/debian lenny main contrib non-free
Update the list of available packages, then install the firmware-ralink and wireless-tools packages:
aptitude install firmware-ralink wireless-tools
Necessary module is rt73usb, check if it is loaded with lsmod. If it is not loaded you can load it by writing modprobe rt73usb.
Verify your device has an available interface:
Raise the interface to activate the radio, for example:
ifconfig wlan0 up
You can check the nearby wireless routers by writing
iwlist wlan0 scan