The GPLed linux device driver called Ralink RT2870 USB ABGN WLAN should be supported since linux kernel 2.4/2.6 serie but isn't included in Trisquel's package archives.
However, Ralink's firmware drivers can be found at Ralink Support for Linux kernel [RT2870USB(RT2870/RT2770 v18.104.22.168 (2010-07-09)]
PS. Reply from Trisquel from 2011-12-07:
> From LICENSE ralink-firmware.txt
> Redistribution. Redistribution and use in binary form, without > modification, are permitted provided that the following conditions are > met:
> The driver is non free, so we won't add it to trisquel.
|SD card readers||51|