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.
ISPacct (formerly Brickacct) accounts the logfiles which Bintec routers spew out depending upon configurable rules. Logfile-analyzing mail accounting functionality is also included (in beta stage). It has multiple backends like text, HTML, and MySQL. It was designed and used by a small ISP and is geared towards such uses as monthly accounting per IP or per dial-in use.
QHacc is a personal accounting program based on Qt. It features double- or single-entry bookkeeping and a reconciliation window. It also provides support for memorized and split transactions, graphing, reporting, archiving, and scheduled transactions that can be based on the account balance.