Probably the biggest untapped server market out there is at the small business level (two to twenty people). At the moment there are a number of Small Business Server (SBS) solutions in this market from a number of companies:
The desktops are the servers
Using Windows file sharing, Samba and Bonjour the business configures their own little desktop-based server for file and print sharing. Thanks to semi-intelligent Internet gateways from the likes of Linksys/DLink that handle DNS and DHCP setting this sort of thing up is relatively easy. Whilst ugly this is probably the most common file/print/Internet sharing solution in most non-tech related small businesses. Unfortunately when things go wrong they can really go very bad, I've seen a number of desktop-servers go belly up because of Internet spyware and viruses.
Microsoft Small Business Server
Micorosft Small Business Server is the most widely used dedicated small business product and by and large Microsoft have (unfortunately) done a pretty good job on it. It is fairly pricey but relatively easy to setup. Microsoft technicians are a dime a dozen (mainly because MSCE's come in cereal boxes these days) which is important because being a Microsoft product it is bound to break thanks to funky software design or rushed out critical security patches. Another positive is that as Windows is the dominant platform support for printers is all but guaranteed.
Linux Small Business Servers
Companies like ClarkConnect produce very tidy small business server alternatives based around Linux. By and large these are pretty good software products that are relatively straightforward to maintain but can be difficult to setup if you are not a semi-experienced Linux head. Products like ClarkConnect have a great feature-set and when they work things go really well but as there is a lack of decent affordable Linux technicians or concise MSDN-like knowledge bases problem solving when things go wrong can be a hit or miss affair.
Novell Small Business Server
Novell do very good 'enterprise' software but compared to the Linux and Microsoft SBS alternatives their SBS offering is too complicated as it is essentially their enterprise software shoehorned into a constrained licensing system. If you know Novell's products and have access to a highly trained (and expensive) Novell engineer who understands Netware or Linux, eDirectory and BorderManager you'll have a system that runs for years without any problems. Unfortunately the overwhelming majority of businesses out their don't have access or the money to pay for such a resource which puts a solution like this beyond their reach.
Where's Apple?
Apple make great consumer level hardware/software but are like fish out of water when put into an enterprise environment. Even their XServe/XSan products are not for general enterprise use and are specifically targeted at the multimedia professionals who tend to favor Apple products. Apple do not have a product that targets the small business server market which is an area where it looks like they could stand to flourish especially as their desktop/laptop sales in this market grow.
OSX Server is a stable, easy to setup and configure server-level operating system. What is planned in Leopard, like calendar sharing, network search and improved Windows Domain support looks like it will be made even better. OSX Server would be ideal in the small business arena as it is easy to setup and can be maintained by non-technical staff with little training. It offers the reliability of a Unix operating system and services like Samba with the ease of use granted by Windows SBS.
Unfortunately OSX Server can only run on Apple hardware and therein lies the problem. The only reasonable hardware platform in a small business environment is the Mac Mini. XServe is just too big for small businesses, it is like hitting a nail with an overpriced sledgehammer. Even the new Mac Pro's are too big and expensive for the low-end server market. Small businesses are not prepared to pay for a high-powered desktop with all the latest bells and whistles just to put it in a corner and treat it like a low-cost Dell. Arguably second-hand Apple gear like a PowerMac or Cube is a great choice but then you run into maintainability concerns plus no-one wants to put their business critical data on a second hand, out of date server if they do not have to.
Supersize the Mac Mini to fill a product void
That leaves the Mac Mini as a viable candidate but size is its major drawback. Performance-wise it is the ideal fit for a small business server where top of the line CPU or graphics performance is not a factor. A larger Mac Mini (for arguments sake called a Mac Serve) capable of fitting three 3.5" SATA hard drives and four sticks of RAM would suit most small businesses expansion needs. The only other hardware addition to the Mac Serve would be an extra ethernet port to handle DSL/cable modems and preferably some extra USB ports to accommodate all the office printers that seem to accumulate.
Cost-wise there would be no reason why it would not be too much more than a standard Mac Mini given the basic hardware and manufacturing similarities. If a Mac Serve/10-user OSX Server bundle cost US$1500 it would be a pretty hard combination to say no to given its ease of use, compact size, features and competitive price when put against Microsoft SBS on a Dell server. What's more the Mac Serve would work out of the box and could in theory be setup by non-technical users who just know they want to share files and printers on a dedicated (and good looking) server that sits out of the way on a shelf.
It is a shame that the chances of something like this coming out are tiny and the likelihood of being able to run OSX Server on non-Apple hardware even lower. It looks like a great small business capable operating system but given the fact it cannot be deployed in offices economically to compete against Microsoft or Linux alternatives it will just stay in the realm of multimedia professionals and Universities, oh well, I guess we can always dream.