Some extreme examples of software stacks are the Linksys routers and TiVo digital video recorders. In these examples, several software components, including the Linux operating system, are combined and distributed as a single appliance. Appliances bring software stacks one step further by distributing the components within a hardware device. Most stacks are not this ambitious, although I would expect that as stacks become more popular, several may evolve into hardware appliances.
And the more commoditized and generic the solution is, the easier it is to create a stack because the individual components in the stack can adhere to industry standards. However, to motivate engineers to build a stack, the solution also requires some level of complexity. If the solution space is relatively straightforward, it would be easier to rebuild functionality rather than try to integrate an existing solution into a stack.
