What's the easiest way to produce an invoice? It seems to be part of larger apps, but what if I just want the invoice capability without a lot of overhead?
The best way to create an invoice is with a spreadsheet, there are a number of spreadsheet applications available in Ubuntu, the main one is the program called calc from the OpenOffice.org Office suite, this is usually installed by default, if you find calc a bit too heavy just to do an invoice then there is the nice lightweight but powerful gnumeric, a gnome based spreadsheet, you can install it by typing sudo apt-get install gnumeric in your terminal or just search for gnumeric in the software center, here is a sample invoice template for gnumeric: http://dl.dropbox.com/u/1035299/Ubuntu_Community/Templates/Sales_Invoice_1.gnumeric
This template can be placed in your Templates folder located in your home directory, any time you need an invoice just right click where you want the file to stay, click “Create Document” in the right click context menu, the item should appear on the flyout menu just click on it and you will have a copy of the invoice in the location you right clicked, you can then rename it and then double click the file, gnumeric will open the file quickly so you can enter your items and costs, this is a pretty swift and lightweight way to ready an invoice for your clients.
Check BambooInvoice or SimpleInvoices. They are both free, open source, web based invoice mangers. You will need to host them on your site or you will need to setup a LAMP server on your machine. If you don't need a server for anything else and won't be allowing external access to it, then you can use XAMPP, which is pretty easy to setup.
I'm a fan of freshbooks http://www.freshbooks.com