The following is an alphabetical list of some of the more useful and popular web services on the Internet, a snapshot of some of the possibilities available at the time this was published. Instead of giving too much detail on every web service, I have categorized each service and provided a brief overview regarding its features. You will find a link to each web service's API page on the Internet, as well as a brief description of the service. I have also defined the protocol each service uses, and given additional information, such as whether an account or developer key is required to use the web service, and, most important, how much the service costs to use. This list isn't intended to be all-inclusive, as the list of web services on the Internet is in constant flux. With that said, you can find a similar list of web services, categorized and with much the same information as I provide in this appendix, at ProgrammableWeb (http://www.programmableweb.com/).
Spam prevention service
Akismet is basically a big machine that sucks up all the data it possibly can, looks for patterns, and learns from its mistakes. Thus far, it has been highly effective at stopping spam and adapting to new techniques and evasion attempts, and time will tell how it stands up.
Free for personal use, $5-$1,000/month for commercial ...