I’ve bought this USB WiFi Adapter at dx.com.

This cheap (5.25€) WiFi adapter has a Ralink5370 chipset,

and works very well with a Raspberry Pi. Just follow the instructions at http://elinux.org/RPi_Ralink_WLAN_devices.

Because I’ve seen some strange behavior (route - default gateway not set correctly) when both eth0 and wlan0 were up, I’ve changed /etc/network/interfaces so that when the USB WiFi adapter is plugged in, eth0 is disabled.

pi@raspberrypi ~ $ cat /etc/network/interfaces
# The loopback network interface
auto lo
iface lo inet loopback

# my wifi device
allow-hotplug wlan0
iface wlan0 inet dhcp
 	pre-up	   ifconfig eth0 down
	wpa-driver wext
 	wpa-conf   /etc/wpa_supplicant.conf
	post-down  ifconfig eth0 up 
# The primary network interface
# allow-hotplug eth0
iface eth0 inet static
	pre-up	ifdown wlan0
	post-down ifup wlan0
pi@raspberrypi ~ $