Vim interface on web browser input fields?

The Pentadactyl and Vimperator addons for Firefox allow vim-like editing in text areas by pressing ctrl+t; I think there is also an option to turn it on permanenly. There is also It's All Text! that will actually launch an editor for you.

As far as Chrome goes, it's not as easy, as the API is quite limited. They require you to run a server on your system. There was TextareaConnect, but it appears to be broken right now.

wasavi provides a vim interface for text boxes in multiple web browsers (Chrom(ium), Firefox/Iceweasel & Opera) and I can confirm that it works on Chromium on Linux (Debian 7 w/ Gnome3). Start it with <Ctrl><Enter>, lots more info on the home page below:

wasavi home page

Github page

for Chrom(ium)

Links for installing to Opera and Firefox/Iceweasel can be found on the home page (link above - I did include them but I can't post with more than 2 links...)

Adding to Shawn's answer, pentadactyl (and vimperator) allow you to press ctrl+i when in an text field to actually edit it in vim. It will save the contents of the text field to a temporary file, open that file in (g)vim, and read the (changed) contents back after closing the editor.