A somewhat belated posting.
Last year I started to port a number of spreadsheets I had into TWiki. I based topics on someexamples I found on the main TWiki site, such as the account ledger which used a "mashup" of the Spreadsheetplugin & the Commentplugin.
The result is a number of different tools: a petty cash balance, a mileage caulcator for expenses; a credit card reconciiation; bank statment reconciliation.
Obviously, I've used attachmentsto hold scans of invoices to us.
It now has an invoicing capability, reproducing the exact layout of our former accunting system. I'm still working on the VAT reporting side of things.
This isn't really boasting about what we've done, it's an example of what we've been able to do with TWiki. Yes, we've had to employ a few plugins (and an addon to import the bank & credit card details), but I feel that we've gained a lot by being able to port our PC based accounting system to a web platform by using TWiki.