*Sites created on or after February 1st, 2016 please read this article on how to donate days to your site.
If you're looking to have your userbase contribute to premium site service, you can use the Crowd Pay widget:
Users can click the 'Contribute' button and make a one-time payment toward the upkeep of the site. Each payment gives a site a certain number of points, which are removed daily to keep the site on a premium plan - the exact plan can be configured on the Billing menu.
Alternatively, you can use the PayPal Donation Widget:
After you've added this widget, there will be an option to 'Config' it (so long as you're still in Widget mode - more information here!) with your PayPal email address. This will let users contribute to the account directly, which you can then use for site upkeep or for server upkeep.