Fortunately most of these sites were inspired by The Hunger Site, and thus have a similar look and feel to them. Notice, however, that many sites require you to click on the advertisement for the donation to occur. Since each web site that you will click on is unique, they all have their own unique policies. A vast majority of them only allow 1 donation from each user per day. In almost all cases, corporate sponsors completely pay for your donation.
Sponsors pay for the donations as a means of advertising and public relations.
To some, this is simply good advertising. To others, it is similar to funding public television or public radio broadcasts, or to making donations to other worthy causes. It provides visibility, an association with a good cause, and public good will towards their company. Visiting sponsors' sites encourages them to continue their sponsorship, and encourages new companies to sponsor free donation sites. If you just choose two sponsors among the hundreds of ads that you will see while donating, this will be a huge encouragement to the sponsors.