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.<p>If you have any question do not hesitate to drop me a line. 
seeit1st@gmail.com