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.

Interestingly, two of the top twenty apps – Digsby and Hellotxt, an application that allows users to update their status across social networks – are not exclusively focused on Twitter. We recently wrote a detailed post on why we cover Twitter-related news so often, and we think the fact that there are so many applications developed as an offshoot of the microblogging system is another testament to the fact that Twitter is not just another startup. This is our attempt to figure out which ones are gaining the most traction.

While we think this is a pretty comprehensive list based on the chosen proxy for popularity, it’s but a first pass at a list we hope will become more authoritative over time. Keep in mind that this methodology is only one measurement of engagement; many people use Twitter iPhone apps and other clients without ever visiting these services’ websites. Derivative site traffic is only a proxy for usage but the most easily measurable one.

We also ran these sites though Quantcast and Google Trends, but chose to measure by Compete because it had the most comprehensive set of data, especially for the smaller sites. So the absolute numbers are unquestionably higher for some of these Twitter apps. If you know additional Twitter apps that should be listed here, or if you have better ideas on how to compile this list, please leave a message in the comments.

It should also be noted that there is a plethora of Twitter application databases out there. Here are a few that we looked at particularly closely: Twitter Fan Wiki, Twitdom, Twapps, Twitstat, and TwitTown.

Update: As many people are pointing out in the comments, Twhirl is not included in this list. That’s because the list is based on Compete data, and Compete measures Twhirl at under 13,000 visitors per month. As we said above, this is just the first pass at creating a list and we certainly realize Compete traffic is not a perfect way to gauge the popularity of applications.

Update 2: So it looks like we owe a “mea culpa” on Twhirl, since we looked up the wrong address on Compete originally (it’s at .org not .com). Twhirl.org actually has 143,000 visitors, putting it at the #6 spot.

Update 3: By popular demand, here is a list of the Top 21 Twitter Clients (According To TwitStat).