Twitter is all about Connections. Oddly enough, in this specific term, they're not referring to the shiny, happy people you've met on Twitter but to the programs in the background doing the heavy lifting and bottle washing.
From the Twitter web interface, click the gear icon in the upper-right corner, next to the Tweet-composition button, choose Settings, then Apps. You'll find a list of all the applications, including official Twitter products and those from third-party vendors, which you've approved to access your account as well as the level of access granted and date given.
This page gives you a handy-dandy button to revoke access for each app.