By Michael Johnston of ETFdbNEW YORK ( TheStreet ) -- The first half of 2010 is in the books, and it is no surprise that the ETF industry has continued to grow at an impressive rate. According to the latest data from the National Stock Exchange, the universe of exchange-traded products saw almost $40 billion of cash inflows during the first six months of the year. From purely a volume perspective, the expansion of the ETF industry has been impressive as well; by our count, the first half saw more than 100 new product launches covering a wide variety of asset classes and investment strategies. Of course, some of these new products have been more successful than others; most ETFs take a significant amount of time to build up material asset levels and trading volumes. But others come flying out of the gates, tapping into a corner of the investable universe that investors are eager to tap but that wasn't previously accessible in an efficient manner. Here, we profile the ten most successful ETFs of the first half of 2010. Our measure of success isn't performance-related, but rather a gauge of which funds generated the most interest among investors. The funds below are ranked in order of cash inflows per day between their inception and the end of June, utilizing the industry data provided by the National Stock Exchange.