I'd totally buy a t-shirt in an interesting design (the Dreamwidth sheep would be a lovely one, if feasible).

Another thought: what about something that entitles people to some sort of special perk - I'm thinking of being in the initial beta test for new features or something of the kind, before they go live to everyone. (Though now I write this, I'm not sure that's technically feasible - but some kind of perk that isn't quite a direct account benefit that I hope someone will have a more brilliant idea for) It'd be a fun way to support the service without dealing with seed accounts.