FlightSim.Com also has a retail page where they sell flight sim related products. They also have a good file library, news, and along with pages for news, and both topic based and product focused forum sections.
Of all the online resources I tapped into for my project the community that hags out in the forums at MyCockpit.org is among the best. The site is jam packed with good information and resources in addition tot he forums and quickly became my go to website for so many different aspects of my build. I probably spent the most time in the sub forum for Link2FSMulti where I learned a great deal about coding for the Arduino and hardware interfaces. I eventually started to share some of my code and build ideas there as well to pay some of what I learned forward.
If you live in the US or are able to travel to the US then the annual FlightSimCon is a great event. It has been taking place in Hartford CT each year in early June for the past several years and gives people a chance to meet rep'd from all the leading software and hardware companies along with hundreds of other flight sim enthusiasts and some pit builders as well.
If you prefer FSX over Prepar3d or your sim is just for recreational purposes FSX Steam Edition is a good option.
SimFlight.com is another good resource for several aspects of flight simulation. SimFlight also has a retail site where they sell a number of flight sim products and a numerous product specific help forums that I frequent.
As a pit builder though the SimFlight help forum I spent the most time in was the FSUIPC help forum. Pete Dowson, the creator of FSUIPC monitors the forum and answers a great many of the posts personally and offers extensive support. A word of CAUTION, Pete will help you at length but there are two things he insists on that, honestly, I agree with and find very reasonable: 1.Always be sure you are running the latest version of FSUIPC; and 2. Read the documentation before you reach out for help.
With the release of version 11 xPlane has become a serious contender. xPlane has always done a better job of modeling flight physics than Prepar3d or FSX, however, their scenery and other functions were lacking and left the user choosing between a very realistic and immersive environment and flight physics. With version 11 they have come up with a scenery system that is well worth consideration and may very well make them the number one pick.
SimForums is primarily focused on Flight1 products and their partner companies, however, the discussion on their forums delve into numerous topic areas of benefit to flight simulation users in general and I've found the answers to a great many perplexing problems in my searches through their forums.
Of particular note on Sim Forums is "THE FSX COMPUTER SYSTEM: THE BIBLE - BY: NickN" it is one of the most extensive documents I've found on how to build and configure a PC and the flight sim software itself. Nick walks you through almost every aspect of component selection and configuration, He covers hardware and software testing and optimization I highly recommend people read and follow Nick's approach when they build there PC.
The good news is there are thousands of resources out on the web for flight simulation and sim pit building. The difficult thing though is sometimes finding them or sorting through the good ones. In addition to the links I've provided on the other pages of this web site the list below gives several others that I found helpful. This is by no means a comprehensive list nor is the absence of any site or resource in anyway a reflection on its fitness or usefulness. It simply means it's not a site I've used or perhaps even one I've discovered.
Avsim is a tremendous resource for almost every aspect of flight simulation. The Library pages and news are extensive and very helpful and the list out forums that a frequent on Avsim is too long to list here. In addition to topic focused sub forums they also have product focused sub forums and some of the add on creators have made Avsim the official home of their help community. For many other add ons users have created unofficial groups and offer some great assistance.
CockpitBuilders.com is another good resource with a very helpful forum community.
After purchasing the rights to FSX and re-releasing FSX Steam Edition Dovetail Games announced it would be using the core of FSX as the starting point to build two new flight simulators, Dovetail Games Flight School and Dovetail Games Flight. Flight School was released earlier in 2016 and as of the end of 2016 Dovetail Games has announced that it is delaying the release of Flight until sometime in 2017.