Dear website owner,

I'm sure you hate spam as much as I do, but I notice you have asked for my email address and I can't see what you're going to do with it. Ideally I'd like reassurance that:

If you're sending a single email - for example, a "send this to your friend" facility - consider something like
We will use the email address to send one email and will then forget it. We will not store it at all.

If you're organizing a time-limited project - a petition, an online game, a one-off event, consider something like
We promise to remove your email address within a week of [your game finishing/the petition being delivered/the event ending].

If you're running a website where the users don't generate notification emails themselves, consider something like
We won't send more than two emails a [month/week/day] to your address.

Having this information prominently displayed next to the signup information makes me (and many other people) much more likely to sign up. This isn't to say that you shouldn't have a full privacy policy as well, just that it's helpful to summarize what you do with email addresses on the page that asks us for our details.

Alternatively, if you don't need to communicate via email and just want a unique login identifier, do consider whether you could allow signup with OpenID as an alternative to asking for email addresses.