How about automatically created anonymous user accounts and ability to later merge into a regular user account with username and password.

An anonymous account would essentially identify a single browser and enable server side storage required for full application functionality.

Should the user later feel like the registration is worth the trouble after all, he could merge one or multiple anonymous accounts into a single regular account.

One thing to think about in advance could be to prepare for merging data from multiple accounts.

I think Amazon shopping cart did something along these lines?