Server Fault has both Internal and External competition.
Internal Competition
These are other Stack Exchange network sites which overlap in some way with Server Fault.
Some are complimentary, others perhaps not so much.
- Database Administrators
This site is narrowly focused on the subject of database administration.
It overlaps with Server Fault in the areas related to configuration of database servers (sizing, storage subsystems, backup procedures, and general configuration of database software like MySQL or Postgres). It also extends beyond what Server Fault normally deals with and covers topics like indexing, table structure optimization, and complex SQL - even "administrative" SQL).
Though they don't say it explicitly, my impression is that dba.SE has similar standards of professionalism to Server Fault -- It's presumed that one is either a database administrator (by training or role) or a very high-end user with administrative access and the core knowledge to use that access effectively.
IT Security
This site is narrowly focused on the principles and practices of what could be broadly called "Information Security". As with Database Administrators it overlaps with Server Fault, in this case in the areas related to the practical application of information security principles (How do I configure X server to mitigate the potential of Y attack?), but dives deeper into the theory behind IT security - why systems work the way they do, how best to apply cryptography, etc.Network Engineering
This site is a personal sore point for me (I'm generally under the impression it has a 100% overlap with the scope of Server Fault, which has always included System Administrators, Storage Administrators, and Network Administrators under its umbrella), but it exists and is currently in Public Beta, so we'll see if it does well enough to stick around in its own right.
The scope really hasn't been fully defined yet, but I imagine if it does well enough to stick around it will be a place for the more esoteric network design questions that require specialized knowledge of the arcana of routing protocols, black-magic MPLS, etc.Unix & Linux
This site is broadly focused on all *NIX-Like operating systems.
If Super User could be thought of as "Tech Support for the (windows-using) Intertubes", Unix & Linux is largely the Unix world equivalent.
The major overlap with Server Fault here is shell-scripting questions and the like, which used to be generally tolerated here even if they weren't specifically about system administration.Operating System Specific Sites: Ask Ubuntu and Ask Different
These two operating-system-speficic sites are dedicated to the Ubuntu Linux distribution and Apple's Mac OS X respectively. They have limited overlap with Server Fault -- most questions asked there are "end user" type questions which would not be appropriate for Server Fault, however on occasion Server Fault may refer particularly esoteric questions to the operating system specific site where the community of subject matter experts might be better able to answer them (for example a question on integrating OS X with a third party authentication system, once it delves into the dark recesses of OS X configuration options, may find a better answer on Ask Different than Server Fault).Other Sites
A number of software-specific Stack Exchange sites have spawned over the years. Many of these questions used to live on Server Fault in absence of a better place for them, but over time those communities have largely migrated to their own sites, and questions particular to these pieces of software (as opposed to configuring a server environment to host them) are often better suited to the software-specific sites. Examples include:
External Competition
Stack Exchange does not have a market corner on Q&A style sites. Expert-Sexchange is perhaps our best known competition (and our grand predecessor, also known for its enormous piles of crap content).
Many forum sites like DaniWeb also have components which overlap Server Fault's scope to varying degrees.
Honestly I don't keep track of our external competition much (because they tend to be awful in terms of quality).