I looked around and found one called OpenBravo POS
. According to its features page, it includes a restaurant module, which may be of interest to you.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:184.108.40.206pre) Gecko/2008101218 GranParadiso/3.0.4pre (.NET CLR 3.5.30729) (ayakawa PGU)
Laptop: HP Compaq nx6325 - Turion 64 X2 @ 2GHz, 2GB DDR2, 100GB HD, ATI Radeon X300, 15" LCD, Seven Pro
Handheld: Palm Treo 650 - Intel PXA270 @ 312MHz, 10MB RAM, 32MB flash, 2.7" LCD, Palm OS 5.4