OpenBravo? Cheap, but powerful. You can buy support if you like, plus your data is in a format that anyone familiar with (MySQL or PostgreSQL) can get at if anything goes wonky. Not like with Quickbooks, or Quickbooks POS.
Thanks for the responses. I've heard a lot about Fishbowl and that they have pretty powerful software. I like that their system also integrates with Quickbooks - I'd hate to have to completely migrate my data if it can be avoided.