ebay (via buydig) has the Sigma 150-500mm f/5-6.3 APO DG OS HSM lens for $724.99 (compare to current sale price of $899; reg. $1069). This deal is a “daily deal” promo on ebay and is available for both Canon and Nikon mount DSLRs. Check it out while it lasts here on ebay.
Sigma is currently offering a $170 instant rebate on its 150-500mm f/5-6.3 DG OS HSM lens, which is available in Canon, Nikon, Pentax, Sony and Sigma mounts. This brings the price down from $1069 to $899. Check it out here at B&H Photo.