I personally use microsoft word for creating and printing all my invoices, contracts, late payment notices, billing, work order forms, etc.
I also use microsoft excel to keep track of client information, client statements, invoices, payments, etc. I just write the invoices down during the day, when i get home at night, i update the spreadsheet. Its a good, no cost system I use and it works very well for me managing my 38 clients.
My set up:
-2007 Chevy 2500hd 8.1l 4x4
-7x16 open trailer
-2011 gravely zt 52hd
-21" craftsman push mower
-Stihl trimmers, blower, edgers, chainsaws.