How do I use MS Exchange 2003 with OS X?
A little amplification of what "apps" you are specifically referring to would be helpful. Assuming your company is using Exchange 2003 SP2, you should be able to use Active Sync with Microsoft Outlook for Mac or Entourage. This should allow you to sync your contacts, email and calendar events. Beyond that an app would need to suport active sync to interact with an Exchange 2003 server natively.