WordPress functionality is extended through the use of plugins. A plugin is simply a program that "hooks" into the WordPress application programming interface (API) that allows developers to write code that changes the way WordPress behaves or interacts with other resources. There are literally over 16,000 plugins available on WordPress.org. This does not include plugins that are for sale only on an author's website. Most plugins on WordPress.org are free or offer a free limited functionality version with a fully featured "Pro" version at nominal cost.
Another issue that appears from time to time is that the source code produced by a plugin does not pass W3C validation. I explained about W3C validation in How to Fix facebook Related W3C Validation Errors, so I won't repeat that content here. But let it suffice to say that many SEO consultants believe that failing W3C validation negatively impacts search engine rankings. With the fierce competition we all face, we need to do everything possible to achieve the highest ranking we can muster. And whether errors impact search engine rankings or not, if the code a plugin produces can't pass validation, it means it is not written to a standard that improves interoperability across a wide array of browsers. Some plugin developers simply don't care about validation at all. They've told me that the functionality they provide is far more important than writing good, validated code. This attitude is not the norm. Most plugin authors I've communicated with really do their best to ensure their plugin is compatible and well written, and they are very responsive to the needs of their customer base.
In order to help you avoid the pull my hair out, throw the keyboard across the room type of troubleshooting sessions that searching for a misbehaving plugin can produce, this page documents plugins that either I or another developer I trust have encountered. If there is a known workaround, it's documented. If it fails W3C validation, it is also documented. We are always at risk when we use plugins on our sites, but the fact is, we need plugins to help us accomplish our vision. Using the plugins on this list increases your risk of something going wrong. But ultimately, it's your choice.
If you've encountered a plugin that either breaks other plugins or fails W3C validation, please report any plugin conflicts you find so your discovery can be posted on this site.