Because we don't make any money off this site and we spend a lot of time working on it to make it better for you. Or because you love the site so much that you're willing to throw a few dollars at it because you're such a nice person.
The donate button. It goes to a PayPal form, but I'm led to believe that you don't need a PayPal account to send a donation.
As much as you want to give. A few dollars goes a long way to helping buy a beer, or some film, or to paying the yearly hosting bill.
You can still use the site without donating. It is free after all.