If you are using Firefox, then the noscript add-on is an option. Most of these ads are loaded by script and noscript will block that. Though this will leave you with sites that depend on script for their basic purpose not working in other ways.
Another option (again Firefox only, though there are no doubt ways to do the same in other recent browsers) is aardvark which lets you manually remove the floaters. I find this handy for removing excess content when printing pages too. This is a manual process, but less of a blunt instrument than noscript. There are some places that are getting wise to such DOM manipulation though, specifically those hawking Microsoft's Silverlight - those floaters seem to detect that you have removed when and they replace themselves (as I've never come across anything on such an affected site that I can't get from many others I just add these sites to my "will never visit again" list, enforced by hosts file entry, and move on).
What you are looking for really is something that automatically does what can be done with aardvark without disabling all script. Unfortunately this is a significant problem as there are many ways to arrange floaters so such a tool would have to carry configuration for each site and would need to support a number of methods of removal, so would be much more of a chore to develop and maintain than noscript and aardvark. Writing something (an add-on or some sort of proxy based filter) to automatically detect and deal with floaters would require a level of AI not currently available (or, at least not currently remotely close to being practical to implement for this purpose!) as there are many ways to implement them and all could be mistake for more useful UI elements so false positives would be a noticeable problem.