Ideally something opensource that runs on linux
I use Xtuple's free Open Source "Postbooks" edition for my accounting / ERP needs on Linux / KDE 4.2.2. It's excellent software and contains everything from invoicing to financial reporting. Make sure you follow their install video which is on the Xtuple website. It's the best free accounting software available today, for both Linux and Windows. If you have any questions about it, please feel free to ask me. Enjoy!