Quasar is a program that offers general accounting, invoicing, etc
and is free in it’s basic form.
It is available, I believe, for both MS Windows and Linux operating
systems. Moneydance is not free anymore, but it is cheap, and
available for free trial. Moneydance is similar to quicken, and
being Java-based, will run on any OS which runs Java.
Depending on the size of your business, it is also possible to track
inventory, income and expenses with spreadsheet programs like Excel,
StarOffice Calc, etc