Another one worth checking out is Appigo Todo. They have a web version (Todo Online) along with versions for Mac, iPhone & iPad. I've been using the iPhone version for over two years and find it to be a great app that supports GTD methodology. I switched from Toodledo to Todo Online earlier this year and am much happier with it. Toodledo is flexible but also cumbersome, has a poor UI and is not good at handling projects.