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
-7x18 enclosed trailer
-Grasshopper front mount 61", gravely 52"
-36" Gravely Walkbehind, 21" exmark push mower