There are a number of variables in a freelancer’s business worth tracking, but the most important is income. It is relatively simple to do and requires only a basic spreadsheet.* You only have to track the actual amount of income, who it came from and when you got it, although I think it’s worth it to combine tracking your income with your invoicing system (i.e. add a column for the date you sent out the invoice).
But why are you tracking these numbers? What are you going to use them for?
First off, keeping track of your income makes the entire income tax process one step easier. Knowing who paid you what means you’ll know when you hit that magic $600 with a given company, so you’ll know when you need to fill out Form 1099 for the IRS, and you’ll keep yourself out of trouble.
Second, since you’ll know when to expect income, you’ll also know when you need to add another income source, and when you can afford to relax a little. Since you’re living off your income, you have to know that you’re making enough to survive.
Third, tracking your income can give you a sense of accomplishment. Knowing that you’ve written something that another person felt was worth payment can give you a warm fuzzy feeling.
*You can even go low-tech with a ledger and a pencil, but you’ll have to do your own math.