We've accumulated a list of the twenty most popular Twitter applications, based on monthly unique visitor data from Compete . Twitpic , an app that lets users share photos on Twitter, took the top spot with 1,236,828 unique visitors in January. Tweetdeck , which came in second with 285,864 monthly visits, is a Twitter app that streamlines notifications and tweets. Third place went to Digsby (with 233,472 monthly visitors), an application that centralizes e-mail, IM and social networking accounts into one desktop program. With 149,812 visits, the fourth most popular app, Twitterfeed , offers to automatically tweet posts published on a user's blog using RSS. A Twitter user ranking site, Twitterholic , is the fifth most popular application, with 147,164 people visiting monthly. (Full list after the jump).