Many small businesses have relied on Microsoft's Small Business Server (SBS) family of servers to get their feet wet with their first server and network. Introduced back in 1997 as BackOffice Small Business Server 4.0, SBS has matured into a tightly integrated platform of the most important services a small company needs: file and printer functions, email, calendar and contact sharing, and document collaboration. While it is limited in the maximum number of concurrent user connections, SBS doesn't shirk core services, providing enterprise-grade features at a price point almost every small business can afford.
The current iteration, Windows SBS 2011, is available in two versions: Essentials and Standard. SBS 2011 Standard (reviewed here) includes Windows Server 2008 R2, Exchange 2010 SP1, SharePoint Foundation 2010, and SQL Server 2008 R2 Express, and it supports up to 75 users. All of the core services -- file and print, email, collaboration, and remote access -- run in a single chassis, but can be extended to additional physical or virtual servers with the Premium Add-on Kit, which is made up of Microsoft SQL Server 2008 R2 and Hyper-V R2.
SBS 2011 Essentials (to be available in the first part of 2011) is intended for very small businesses with up to 25 users. Essentials provides a file and print platform for local applications while integrating with cloud-based email and collaboration. Unlike SBS 2011 Standard, Essentials doesn't include a copy of Exchange or SharePoint Foundation on the local server. All email and collaboration is done in the cloud through services such as Microsoft's Office 365. The physical server runs a full copy of Windows Server 2008 R2 and handles user authentication and server management.