Something that is probably unethical, but has worked well for me.

On the Marriott.com website for each specific hotel property, "nearby businesses" are typically listed along with the hotel amenities, restaraunts, etc. It seems logical (and my experience has shown it to work) that these nearby companies would be likely to have special corporate rates with the hotel.

As mentioned previously in this thread, its usually pretty easy to guy the 3-letter code for most big companies (FED=federal express, COK=Coca Cola, BOA=Bank of America, etc.) I usually run through the list of company and drop in 3 digit codes and typically find one that works pretty well. Just this week I got a room for $89 that would have been $159 with no code.