Event Tickets Plus is a WordPress plugin you can use to sell tickets for events.
Part of its feature set is a natty bit of functionality called Custom Attendee Meta, which lets organizers collect arbitrary bits of data from attendees. For example, it can be used to gather information such as t-shirt sizes or special dietary requirements … and so is incredibly useful for many events.
At time of writing, though, it does not yet allow the site admin to edit the submitted data. So, if Tom Thumb comes along and buys a ticket to an event and incorrectly gives his t-shirt size as medium then sends the site owner an email requesting they update this to extra large, the site owner is out of luck — they’re going to have to come up with some other system for recording this change (unless they poke about and make changes directly within the database, something often best avoided).