The Plaza and Las Vegas Club (downtown casinos) usually send out quarterly mailings which include free rooms and other amenities. The upscale strip casinos aren't as generous, but for weekday rooms, they routinely have very good "casino" rates. About two years ago, Terribles (off the stip) was charging $9.00 per night for early december (one of the slowest periods in vegas.)
Signing up, by giving your e-mail address, will frequently get you nice offers.