Recommend a web-based e-mail service, please

I’m thinking of signing up for a web-based e-mail service. (Yeah, I know, but I haven’t had a reason to.) I’m experimenting with it as an address to use when I strongly suspect I’m going to get marketing material in exchange for merchandiser discounts, frequent-eater plans, etc.

Any recommendations on what service to use? What criteria am I looking for?

Edited – BUMP!