In Working on tasks in the right context, I already mentioned ways of managing to-do-lists online. I'm using the more universal Backpack, but there also are web services that are especially geared towards the "Getting Things Done" method of task management.
Another free service is iCommit which also uses the GTD nomenclature of tasks, contexts, projects and so on. To me it doesn't look quite as smooth as Zenlist. That may be one of the reasons that iCommit is getting a complete overhaul at the moment. But it undoubtedly does the job, too.
I'm looking forward to more productivity tools on the web. The beginnings certainly are promising.