Coda for OS X
I am not sure what I would do without this amazing application on OS X. I have found myself using this application so much this past month that it can only get worse. Now remember, I have been using this application since it came out, just not as this much as I have this past month. If I had stats on the usage, I would post it.
I do not just use Coda for my web coding though. I have found myself using it to write huge replies on forums that are using terrible scripts that are not like Invision Board or vBulletin or replying/writing new private messages on forums and what not too. So it comes in handy for that as well.