GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.
Ledger-cli.org - Ledger is a powerful, double-entry accounting system that is accessed from the UNIX command-line.
Xero - Xero is online accounting for your small business.
Teamleader - CRM, invoicing & project planning in a single tool for SME's
CheckStream - CheckStream check printing software
Free Invoicing Software - Free Invoicing Software provides the facility of online billing.