Running a business for yourself means you have to be inventive and always on the lookout for a new and better way to get things done. Innovation junkies, take note: the Internet has a lot to offer. From invoicing to marketing, these are tools that freelancers need to know about.
Some great suggestions here, but I don’t know if I would call it “everything you will possibly need”. For example, what about an internet connection? …That’s pretty important for a web app to function right? :P