InGroupOffice we have categories in tasks and calendars. The categories can be defined independently and you can filter tasks by categories.
Outlook and Thunderbird users often mark mails by categories, usually then they get a different colour. With Thunderbird, you simply press 1 or 2 or 3 to categorize messages, when using Outlook with Exchange / MAPI then the categories get also synced.
I would appreciate a similar feature in GroupOffice mail. Unfortunately, the IMAP protocol does not support syncing categories. Some workarounds are explained here: http://www.msoutlook.info/question/471
Inserting the category in the subject of the mail may not be feasible, as you need to tweak a message. Splitting up categories into IMAP folders would be the only solution, but you loose the common view in one timeline. Create an e-mail view with joined folders? Any other idea?
Furthermore it would be nice to be able to search all elements (tasks, calendar entries, mails) by categories. For example show me all which is related to my urban gardening activities.