ddg have their own index, and then defer to bing (yahoo also uses bing for results, so you could argue they are yahoo results) what they do differently is to query lots of 'authoritive' sources to try and provide a good result without an index. so if you search for 'perl fork', your first result will be linking you to 'fork' in perldoc.com it also plugs in to wolfram alpha to provide answers to humanish queries like 'what is four times five', currency conversions etc. so yeah, they have stitched together a *LOT* of third party apis (especially free ones) to bring you results. its the sort of approach that will polarise peoples opinions, with people going 'thats lame they are just using apis... boo!' and other people saying 'hay cool, thats a great - why hasnt someone done that before' either way, since its a free website if you dont like it you can easily get a refund.