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 backed by an active development community and is blossoming into a full-fledged accounting system.
The Cybercafe Organizer (CybOrg) is a point of sales and administration system for Internet cafes (cyber cafes). It has a Web-based interface and is written in Perl using Template Toolkit and a RDBMS. It uses a client to block workstations, and is intended to be used on a (possibly Linux) server with Win32/Linux clients.It also provides a script to be used as the Active Desktop on Windows clients to display station's status.
kivitendo, formerly known as Lx-Office, is a Web-based accounting system tailored for the German market (e.g., tax calculation). It can export accounting data in the German DATEV format and features invoicing, sales orders, quotes, etc. Forms are generated on the fly as PDF or OpenDocument documents.