UPDATE 1-OpenX launches new online ad marketplace

(Adds background on advertising market)

By Georgina Prodhan

LONDON, April 16 (Reuters) - Technology start-up OpenX launched an online ad marketplace for smaller Web publishers on Thursday designed to be an alternative to offerings from Google (GOOG.O), Microsoft (MSFT.O), Yahoo (YHOO.O) and AOL (TWX.N).

OpenX Market aims to make it easier for advertisers to find smaller Web publishers as the number of sites run by individuals and small organisations with niche audiences balloons, and to help those small publishers maximise their advertising revenues.

OpenX, backed by Index Ventures and Accel Partners, runs more than 300 billion page impressions a month through its software from a network of about 150,000 websites, putting it in the league of the likes of Google's DoubleClick in terms of volume.

Google bought DoubleClick for $3.1 billion in 2007, part of a multi-billion-dollar wave of ad-server company acquisitions that included Microsoft buying aQuantive, AOL buying Adtech and WPP (WPP.L) buying 24/7 Real Media.

"Now the space is very concentrated, which makes it really good for us to be an independent option in the market," OpenX's Chief Executive Tim Cadogan told Reuters.

Cadogan, an industry veteran who formerly ran Yahoo's search business, said OpenX's open-source software that publishers can easily customise was another attraction.

OpenX's new platform stages real-time auctions for online advertising spots, allowing publishers to take advantage of higher bids until the last second. It is likely to appeal most to small publishers, but scalable for large enterprises too.