I suggest using this for devs or advanced users only. Ones that won't freak out completely if their browser crashes and they loose some data. This is solely because Text Area Cache is way better than Lazarus on Chrome.
Salesforce.com Sandbox Favicon - So useful. Firstly it adds an S on the favicon when you are logged into a Sandbox - so helpful! Secondly it adds the instance of Salesforce to the titlebar (eg if you have a custom URL it will show (customurl) in front of the regular title.
Salesforce.com Enhanced Formula Editor - I find this one both totally annoying and totally useful. I can't decide whether to have it turned on all the time or not. Whenever I have it turned off I miss it, when I have it turned on it annoys me. New features mean that it is less annoying though, so worth having.