Journal pbhj's Journal: POS software
http://www.fisterra.org/ - options to gear specifically for a garage environment
http://www.jpos.org/ - looks like a development framework for Java pos systems, not even sure this does point-of-sale
http://easypos.sourceforge.net/ - usable for cash-till only, looks pretty naff!
http://sourceforge.net/projects/custom - good list of features, may need lots of work to get up and running effectively
http://europos.sourceforge.net/html/screenshots.html - big bold pub style POS system, just does the cash-till bit and a little MySQL accounting, derived from L'ane POS
http://l-ane.sourceforge.net/ - the best looking of the bunch, this handles PostgreSQL and has a small interface for backend stuff.
Perhaps integrate l-ane's POS with sql-ledger's accounting facilities, might need modification of L'ane to write to sql-ledger's DB?? Of course there's the bananahead stuff too, which might be worth another effort.
LinuxCanada offers a further option with QuasarPOS, looks pretty cool. Single license is free, addons are per-pay. Does this use an accessible DB though?