Over the last couple of days I have come across three very useful tools:
Google Browser Sync
Just released it keeps all your Firefoxes in sync with each other. Great if you have more than one computer (or virtual computer). I have used other bookmark management tools in the past but as this is integrated right into Firefox, automatic and free it is hard to beat.
Letterbox
Widescreen view for Mail.app provides a three panel view very similar to that of Outlook or Thunderbird. When you have a widescreen display it makes a lot of sense.
BlueHarvest
If you have ever browsed network shares (i.e. Windows/Netware/Linux shares) with OSX you will know that it leaves a lot of debris scattered around the remote filesystem (in the form of hidden files). Whilst invisible to the casual OSX user they usually stand out to Windows and Linux users and are even sometimes used to explain why Windows is better than OSX (figure that one out). BlueHarvest runs around behind you cleaning up the mess so to speak. You can also get it to search places you have been to in the past to clean up.
Also today my Google Spreadsheet invitation came through. It looks very promising but it really needs to leave the Labs and get integrated into GMail/GTalk/Search in the same manner that the 'View as HTML' function has become for Word, Powerpoint and PDF documents.