If you have no more than 3 clients you can still use FreshBooks for free.
If you have between 4 and 25 clients then you need the FreshBooks Seedling Plan that costs $19.95 per month and for more than 25 clients then you'll need the next plan up which is $29.95 per month and then the last plan for even more clients is $39.95 per month.
FreshBooks is a great software and well worth the money if you need the higher plans.