GnuCash is personal and small-business financial-accounting software, freely licensed under the GNU GPL.
Designed to be easy to use, yet powerful and flexible, 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. It is available for GNU/Linux, BSD, Solaris, Mac OS X and Microsoft Windows. Read more.
Alternative apps to GnuCash:
Money Manager Ex is a free, open-source, cross-platform, easy-to-use personal finance software.
AceMoney Lite is a free personal finance software, the best free Quicken and Microsoft Money alternative. All features from its big brother are there, limited only to two accounts.