Advanced filtering functions for WordPress, including the Talk Like a Pirate Day filters.
The Text Filter Suite ("TFS", hereafter) is a WordPress plugin which adds some new text filtering functions. In a sense, the core TFS functions could be considered a "meta filter", because they actually provide a framework that let you construct new filters fairly easily. They also provide an easy way to apply filters to post content and comments on a per-post basis.
Okay, let's try a real-world example. A common feature on many web sites is the automatic handling of acronyms. You'll often see an acronym such as "XHTML" displayed in an alternate style, and when you hover your mouse pointer over it, you get a tool-tip which displays the definition ("eXtended HyperText Markup Language", in this case). You don't want to have to type in the markup for this every time you post something, and automation is what computers are for, right? So, there are several plugins available to handle this sort of automated text subsitution.
TFS comes with the "TFS Acronymit" plugin to perform this function. It's based on Matt Mullenweg's original "Acronymit" function, but with a couple of improvements. In Matt's original function, you had to keep the acronym list sorted, longest-to-shortest, and it could get confused by recursive acronyms like 'PHP' ("PHP Hyptertext Processor") or GNU ("GNU's Not Unix"). TFS Acronymit does not have those restrictions.
For those who think that expansion of technical acronyms is "teh sux0r" (i.e. "boring"), TFS comes with a variety of more entertaining filters. The current set includes "chef", "fudd", "jive", "kraut", "pirate", and of course, the aforementioned "acronymit". As a word of caution, the "jive" and "kraut" filters are not what you would call "politically correct".