Most simplistic and fastest add-on for generating documents from a template. This add-on is simply a MUST for anyone who want's to use google docs for business. An unexpected GREAT and UNDERadvertised feature of this app is:
the user can partially fill out a template. The fields left blank will be left alone as variables (meaning that you can use a template to generate a more specific template, and fill in the rest of the variables later).
For example, you can create a general "Invoice" template (with fields such as customer name, address, account, PO#, order desc, shipping, and $total), and if you have one customer who comes often, just fill out their info (name, address, account number, etc) and leave the rest of the fields blank (such as PO#, order desc, shipping instructions, $total). Now you have an invoice ready to go for that particular customer!