I am sure that all the programs mentioned here are useful and helpful. However, the best open source accounting software by far is GnuCash ( www.gnucash.org ).
I have been using it for years to do the books for online retailers and hedge funds and still get amazed by the versatility of the program.
If you have any question do not hesitate to drop me a line.
[email protected]
https://www.youtube.com/watch?v=Ut9GdMywFj0