It's never really managed to gain much traction, but I've long been fond of Flattr's model: you allocate a monthly budget, and that's then split across all the sites you've flattr'd that month. (If you really liked something, you click more than once, to give them that many more slices of the pie)

That way, you know how much you're spending every month, avoiding the tip jar situation where you might wind up donating more than you'd reckoned, and you're able to contribute, even if modestly, to that many more sites than with, say, Patreon.