New York—The Wall Street Journal Online, the largest paid subscription site on the Web, celebrated its 10th anniversary Monday. To mark the anniversary, WSJ.com is hosting an open house through May 10, during which access to the entire site will be free. Charles Schwab & Co., Hewlett-Packard Co. and Sprint Corp. will sponsor individual days during the open house. Each sponsor will control most of the site’s impressions that day and will be integrated into the open house promotional advertising, which includes ads in The Wall Street Journal, MarketWatch and Barron’s. As of the first quarter, WSJ.com had a paid circulation of 761,000 worldwide.