People keep asking in our forums* for better ways to spread the Sponge. Here's a start:
Write a review on Amazon. I'm sure there is some sort of revenue sharing available.
del.icio.us us. gross.
Vote for Spongecell on the web2.0 List. The description of Spongecell as "Email or User Id. Password. Remember Me." is dead on.
Vote for Spongecell on Listable
Review Spongecell on BuzzShout
The best thing you can do is tell your friends, maybe show them how easy it is to add new appointments from your phone.
Also, I bought a couple hundred sponges on ebay, they were supposed to be for the launch party but didn't arrive in time. Email me if you have any bright ideas about what I should do with them – marc at spongecell.com. Maybe we should have another party? 
*The forums are currently in an invite-only beta.