I'm typing this while on a break from data entry into Solomon IV, another Great Plains accounting package. I have to enter invoices by hand, because the import program doesn't work. Overall, my Dad's company has spent $100,000+ and more than a year trying to get it to work for a relatively small service company. It crashes at random intervals, the user-interface is cumbersome, and the software makes it difficult to make changes once you've entered the data.
Unfortunately, I don't believe any of the current crop of open source accounting packages could meet your needs off the shelf. I believe the closest may be the Linux-Kontor project. You should also check out Christopher Browne's canonical list of links to open source accounting/finance packages. For a small company, SQL-Ledger is also worth a look.
Personally, I would love to see someone create accounting modules for the ArsDigita Community System. It is a web collaboration toolkit
initially developed by Philip Greenspun, Tracy Adams, and others at MIT. It is now currently maintained and extended by the developers at the ArsDigita Corporation (250+ mostly MIT/Caltech trained
programmers), as well as a healthy external community of developers. Currently, there are two versions of the ACS: ACS "Classic", which runs on Oracle, and OpenACS, a port to the open source PostgreSQL database.
The ACS has been used build a number of high-volume, high-security web services for large companies including Oracle, Siemens AG, and Hewlett Packard. The ACS was used to build an experimental "pants-tracking system" for Levi's (though Levi's ultimately decided not to make the system live.) Other projects include an electronic medical records system and a site for selling municipal bonds. (See case studies page for more details.) Also check out http://www.arsdigita.com/pages/toolkit/modules.adp.
for a list of the modules currently available.
It is extensively documented, and training is widely available. Among the resources:
Philip Greenspun's book "Philip and Alex's Guide to Web Publishing"
(200+ five star reviews from Amazon.com; also completely available
Complete course content of MIT course 6.916: Software Engineering of
Innovative Web Services, which teaches you to use the ACS, is available
http://philip.greenspun.com/teaching/psets/ps1/ps1 This course has
since been adopted by Stanford, Berkely, Caltech, and a number of other
Free 1 - 3 week boot camps taught by ArsDigita employees:
Detailed installation guide for both OpenACS:
http://www.openacs.org/doc/openacs/index.html and ACS Classic:
Procedure by procedure documentation:
Note, I have no association with ArsDigita, other than as a fan of the company and software.