For businesses it's essential and to keep a good track of your personal expenses it's very useful: money management software.
Quicken is probably the most famous accounting software. But it's not open source and even the basic solutions offered can have a quite steep pricing. In this article we'll focus on open source solutions.
GnuCash
GnuCash offers double entry bookkeeping with a checkbook-style register, scheduled transactions and reports and graphs. It's
Odoo
Odoo is a suite of open-source business apps with a lot of features.
More information
Wikipedia has extended information about financial software packages.