From “Things public preview released“:

Things provides a unique and flexible way to handle tasks that can be refreshing for those who didn’t quite get the whole GTD thing the first time around. Of course, you can adhere to strict GTD principles with it, but you can also forge a system that works for you using its tagging system and flexible project management tools.

For those not familiar with the concept, what distinguishes GTD from other time- or action-management systems is the idea of grouping tasks by the context (defined as a place or set of available resources) in which they are to be performed.

Update: Marc-André pointed me to Thinking Rock as a FOSS and cross-platform alternative. It’s not looking half bad. Gotta try it out too.