It's certainly a common question, and while "buy them as you discover you need them" is not a bad strategy -- especially since the answer depends on what kinds of projects you want to tackle -- there are some basics, and some things which are more useful than they may appear at first glance, and those which you can live without but which really do make the job so much easier that...
But this is definitely an opinion/poll kind of question, and I don't think we want to open that can of worms right now. If ever.
On the other hand, if someone has a really good reference for this.. I know SE dislikes offsite pointers, but it might be good to have a canned answer on the site just to let us refer folks to it.
Just a thought. May be a silly thought.