Good Program WLAN Calculate Real Speed.

Hi everybody,
I'm searching for a long time a good program (maybe on GNU license) which
could be able to calculate the "real speed" of my Wi-Fi network.
If this program exist under Windows should be nice. For my Signal test i
already usse Netstumbler which i think is very good.
I discover Wifi Manager 4.0 which is free too but i can't visualizate Speed
traffic.
So if someone have it one which don "server" on the Wi-Fi computer and
generate and calculate traffic throught another PC should be nice !
PS : I need one wich do it in "virtual" to be sure the speed results are not
falsed due the hardDisk.
Thank you in advance