Wren effectively is Birdhouse for the Mac, except that it lets you keep just three drafts. Regarding this limit, the FAQ says:

We like the idea of constraining the number of drafts. It helps keep focus on tweeting and not managing a huge list of potential tweets. We may eventually change our perspective, but we’re pretty set on this.

OK, but I still don’t get it. Why shouldn’t the user be left to decide how many drafts they want to manage?

Also in the FAQ they entertain the idea of making an iPhone equivalent. Yes, please! But, I hope they don’t stop there. What they should do–especially if they like making money–is remove the draft limit, create the iPhone app and have the two apps sync drafts via Dropbox (or similar). (I, and countless others, have long expressed our desire for Birdhouse to sync to anything, but it seems that request will forever go unfulfilled.) There’s serious money to be made here.

Relatedly, if you use LaunchBar, you might consider the script I cobbled together for tweeting via LaunchBar. It’s not as pretty (and wow, Wren definitely is pretty), but it is free.