Ask HN: Web app pricing... to charge or not to charge

I'm avoiding the temptation of delaying my web app's release due to lack of features and polish. As I've read on HN over the past 6 months: Release early! then iterate... (I've drank the kool-aid) I will be releasing my application sometime in June. (yikes)

My next concern regards whether or not to charge users to use my application. I've read articles and have had debates over the past few months. Now I'd like to pose this question to the HN population:

Assuming the founders' costs are are trivial (excluding time), what are the scenarios/reasons to charge users to use a product vs. providing free access to all users?