I got recently a Ralink RT2870/RT3070 USB Wireless Adapter which is available in the mainline kernel, but unfortunately the firmware package does not exists yet in Portage. Firmware is available from http://eng.ralinktech.com.tw/support.php?s=2 and "Firmware RT28XX/RT30XX USB series (RT2870/RT2770/RT3572/RT3070)" worked fine for me.
net-wireless/rt2870-firmware was added to portage a few days ago.