Do you do your own taxes or have an account do them? If you hire that out, I'd talk to your accountant about what different categories he needs from you at tax time. Start with those, then add as you see fit.
Without going to look, I think I have:
*1 for each vehicle (could put all into one for tax purposes, but it will make it easier for checking maintenance records, etc. if they are separate)
*Fuel (I actual expense my truck, so that is written off, if you report milage to IRS, the vehicle's fuel cost does not matter, but chainsaw gas certainly does!)
*Large equipment (that you will depreciate)
*Small equipment (one time write off)
*"resale" purchases (for example, if you do pesticide applications keep the chemical receipts here; the cost of trees I plant go here too)
*Insurance
*Workers comp
*Supplies (postage receipts, office supplies, etc...some may separate, but I am not running a major office operation here! Could also put all the other expenses that are written off here ie: chainsaw gas if you want to keep that separate from a vehicle fuel folder)
*Misc expenses (This is where I keep things that cannot be written off of the taxes, but I still need to track...for example: chainsaw chain, files, work clothing, cell phone bill...I pay with company money, but use it for personal use as well so would be a tough sell to write that off, etc...)
*Bank statements
*Deposit records (I keep those and 'bank statements' in one folder, but there may be good arguments for separating).
*Tax records:
--Federal withholding
--State withholding
--Minicipal withholding
--Federal SE tax
--Sales tax filings
--soon to be added health care tax
*Open contracts
*Completed contracts
*Pesticide application records
*Pesticide licensing info
*Cert. Arborist info/continuing education hours
I also keep a folder of "field notes". If I really wanted to go nuts, I'd keep a seperate file for each client that would have notes, contracts, etc... in it. I do this electronically...but there is some logic to keeping hard copies - but I am not organized enough to maintain that.
I have a spreadsheet tracking project completion, billing and payment, so I don't have folders for those.
I am probably missing a couple, but those seem to be the ones I am putting stuff in most often.
Look forward to see what others have to say.