This page describes how you can make the original Ralink RT73 (aka RT2571) drivers work. This page does not describe how to make the new from-scratch drivers written by the rt2x00 project. (For rt2x00, visit rt2x00 wiki page)
Several USB WiFi dongles use this chipset, including the Linksys WUSB54GC and the Belkin Wireless G USB key (F5D7050 v3000)
RT73 has support built into the kernel since 2.6.25, the steps below may no longer be necessary.
However, some RT73 devices (eg: Linksys WUSB54GC) need to load the firmware, or they won't really work (you can find the firmware requests by dmesg command, eg:
dmesg |grep rt73). So you have to install the linux-firmware package, using:
pacman -S linux-firmware