You may want to list out the requirements of your applications and then look into the numerous features of the SharePoint 2010 editions here - Foundation/Standard/Enterprise.
The Foundation edition is not "free" - it is added onto a Windows Server or Win7/Vista (all OS are paid for). SharePoint deployment on Win7/Vista is for development purposes only (not for production deployment). More on it here - Setting Up the Development Environment for SharePoint 2010 on Windows Vista, Windows 7, and Windows Server 2008. All editions have an additional hardware requirement of 64-bit operating systems (& SQL Server).
Technically, the primary differences between the editions are with the various service applications & their available features, for e.g., Business Connectivity Services, Managed Metadata, Search, User Profile, Office applications, PerformancePoint (BI) and others. See this diagram for more details regarding - SharePoint Services (top right corner table lists services features & editions).