WebCake is a browser extension that shows ads in Internet Explorer, Safari and Firefox. In the screenshot to the right you see it showing a banner ad at the bottom of the page and a floating coupon ad to the right when browsing amazon.com.
If you see ads labeled "Webcake Ads" while browsing the Internet you have the WebCake software installed on your computer. You can also see it in Firefox's Add-ons tab and Chromes Extension tab as the two screenshots to the right shows.
You may also see files such as webcake.exe, WebCakeIEClient.dll, OptChrome.exe, WebCakesetup.exe, WebCakeDesktop.exe and webcakedesktop.updater.exe running on your system or on the hard-drive.