Microsoft offers two portal solutions, Windows SharePoint Services (WSS) and SharePoint Portal Server (SPS). For most mid-sized organizations, WSS will provide the necessary features and functionality.
WSS: Free with Windows 2003 Server
WSS is the successor to SharePoint Team Services and is available as a free download to any organizations using Windows 2003 Server (Web, Standard, Enterprise, or Datacenter Edition). WSS only runs on Windows 2003 and requires Internet Information Server (ISS) 6.x, ASP.NET, and the .NET framework 1.1. It can utilize SQL Server 2005, SQL Server 2000, and MSDE 2000 as data repositories.