For businesses with small, single offices to large enterprises with racks full of Xserves, Snow Leopard Server makes a lot of sense. That $499 license provides a lot -- a mail server (not Exchange-compatible, however), CalDAV server for group calendars, group-wide Time Machine backups to a server, a podcast producer, a wiki server, an iChat server, and the new Mobile Access Server, in addition to a plethora of other administrative and client management options.
Along with the Xserve, which I've found to be comparably priced to other enterprise-class servers, and the Mac mini, which makes a great little inexpensive headless server for small offices, Apple has created a set of powerful business tools. Considering that it's now half the price of Leopard Server, Snow Leopard Server is something that more system admins may want to look at, especially in Microsoft-dominated shops.
Apple Mac OS X Snow Leopard