I recently adopted Gnucash as an alternative to my old faithful expenses spreadsheet using Gnumeric and I have to say, after grasping the basics it improved my bookkeeping drastically. One very useful and brutally simple feature that I like is the ability to attach files to a transaction. In Brazil we use boletos[1] extensively and Gnucash made my life a heck of a lot easier to keep track of them.

[1] https://en.wikipedia.org/wiki/Boleto (roughly translating, a ticket)