i've found some useful images on morguefile (mainly outdoor landscape shots...) but based on need, i use dreamstime at the office and usually pay for the "better" shots , same with stock exchange (sxc.hu).

these sites are great for photographers starting out in the "stock" trade, as they offer the ability to get exposure (no pun intended) as skills get better. 

those are the three i use the most

:)
retrocity