GeniusTrader is a perlbased, GPL-software for the creation of trading systems and to backtest them. It also includes a large database of common indicators and signals and is capable of displaying all the stuff in various formats.
The GnuCash Project aims to develop GPL'ed financial/accounting software modules. GnuCash itself is a Gnome Desktop application aimed at personal finances and home users.
An API that allows applications to support Open Financial eXchange command responses, usually provided by financial institutions for bank statement downloads.