When PayPal filed for its successful IPO last year, it listed a slew of competitors in the online payment arena. But those rivals, including Citibank, Wells Fargo, Yahoo, Microsoft and the U.S. Postal Service, barely sport as much market share combined as PayPal commands on its own. There are signs that some companies still see plenty of opportunities in certain subsectors of this industry. Could they succeed, or has PayPal so dominated the market that would-be startups are destined to failure?