Kasper Nauwelaerts has released a small plugin that lets you validate the webpages you browse for (x)html compliance. In the bottom right corner of each browser window, it shows the number of warnings and errors the current page has, along with a small icon. Clicking on the text shows the source for the page, along with a list of all the problems. Double-clicking on the error message in that window highlights the problem within the source code. Invaluable!
(via Lars Pind)