GNU Cash

GnuCash is personal and small-business financial-accounting software, and is available for GNU/Linux, BSD, Solaris, Mac OS X and Microsoft Windows.

Designed to be easy to use, yet powerful and flexible, it was initially aimed at developing capabilities similar to Intuit, Inc.'s Quicken application, but also has features for small business accounting. 

GnuCash allows you to track bank accounts, stocks, income and expenses. As quick and intuitive to use as a checkbook register, it is based on professional accounting principles to ensure balanced books and accurate reports.