Flashing the ASUS WL-520GU to Tomato

Here.

See also the DD-WRT wiki. The relevant ASUS firmware restore utility is here. You want dd-wrt.v24_std_generic.bin for the first flash and Tomato-ND.trx for the second flash. Don't forget to reset the password when you're done.

Update: Use the latest Tomato with VPN patches from here