PlanHammer

This application (not just a website – TOTALLY an 'app') is heads above other planning systems. Here's why: this app gives you access to robust tools while keeping the application and use of the tools simple. Amazing. Plus: support is spot-on and ready to help.

Very cool concept and I love the interface but I have encountered a bug that caused a number of items to vanish. The developers are currently working with me to fix the problem. Will update once it's fixed, until then if you need to change the hierarchy of some items, don't do it from the list view.