Sponsoring online is a form of paid advertisement. Consider your targeted market when choosing where to become a sponsor.
Direct Advertisement Sponsorship
Your site's link or banner will be posted and viewed by many when you sponsor any of the following:
Contest / Event Sponsorships
Contest Sponsorship - donate a product or service giveway for another site's contest. Choose a site that has the same target audience as yours to get the maximum return. The site's contest page should state that your company is sponsoring the contest with your company's logo and a link back to your site. This information should be part of their email announcements, the contest page on their site and any form of marketing their contest. Most of the time, you just get the recognition only. Occasionally, you can negotiate to also receive a copy of the emails gathered for signing up for the contest.
Event Sponsorship - pay for all or partial costs of an online (or offline) event; such as a webinar. In exchange for covering costs for an event, the event marketing should include your company's name, logo and site's link. Your company should be mentioned during the event as the sponsor. Depending on the event, you may have a few minutes to speak as the sponsor.