Popular Twitter client MeTweets is currently available from the Windows Phone Store for free. Normally priced at $1.49, the app has been free since April 1, and the developer hasn't specified how long it will remain free, saying only that it will be free for a while.
MeTweets has all of the features you would expect from a Twitter app. The app supports voice commands, tweet dictation, and inline images from OneDrive. MeTweets will also let you post tweets longer than 140 characters with its SmartSplit feature, which automatically splits long tweets up, then merges them again in your timeline for a single, continuous reading experience.