There are many software or web service comparison sites out there, but Iterating has managed to provide a fully-loaded site that has a number of useful search filter options. You can use their matrix feature to compare similar web services, and you can look through their wiki database of web services using various filters, or just by looking at the highest rated ones, recently posted ones, or a specific category. The site is well-organized and incredibly detailed. Each web service is listed along with its critical information and user reviews.