You can download Windows SharePoint Service 3.0 (WSSv3) from the Microsoft Download site.

Note: Installing Microsoft Windows SharePoint Services 3.0 over any beta or trial edition of Microsoft Office SharePoint Server 2007 is not supported.

Windows SharePoint Services 3.0 Windows SharePoint Services is a versatile technology that organizations and business units of all sizes can use to increase the efficiency of business processes and improve team productivity. With tools for collaboration that help people stay connected across organizational and geographic boundaries, Windows SharePoint Services gives people access to information they need.

Built on Microsoft Windows Server 2003, Windows SharePoint Services also provides a foundation platform for building Web-based business applications that can flex and scale easily to meet the changing and growing needs of your business. Robust administrative controls for managing storage and Web infrastructure give IT departments a cost-effective way to implement and manage a high-performance collaboration environment. With a familiar, Web-based interface and close integration with everyday tools including the 2007 Microsoft Office system, Windows SharePoint Server is easy to use and can be deployed rapidly.

Here are the Product Keys you will need when installing SharePoint Server 2007

SharePoint Server Standard Trial: XJMKW-8T7PR-76XT6-RTC8G-VVWCQ
SharePoint Server Enterprise Trial: F2JBW-4PDJC-HKXTJ-YCKRP-T2J9D

Other SharePoint related downloads: