How can I categorize the apps on my phone? I'm downloading many apps but then forget about them. I'd like to be able to see all my "note taking" apps, "todo" apps etc. So I can quickly compare and discard those I don't want. Of course I'd also like all "game" apps so I can choose which game to play.

How can I do that so it is automatic (maybe use the information in market?). Is there a way to tag apps so they can belong to several categories?
There is an app called Apps Organizer that will let you "tag" applications into groups.  You can then put folder widgets on your homescreen of each of these "tags" to stay organized.  I've used it before, it works well if you keep your tags up to date.