I know there are very few geeks that are interested in such beasts, but way back when there were one or two geeks that used to dabble, namely @Darnit696 and @anon96558872.
Microsoft Flight Simulator X (FSX) used to be king, but since Microsoft canned their flight simulator division after their horrible attempt at appealing to the masses with the now defunct Flight, FSX is no longer updated and is a pure crashfest on Windows 8 and up.
Prepar3d
An alternate to FSX has popped up, namely Lockheed Martin’s Prepar3d, which is based on the FSX code. They have updated it and made it compatible with the new versions of Windows - apparently most of the third party addons for FSX work with it either directly, or via a shim, and some third parties are actually developing mods for it. HOWEVER, the big caveat is that Prepar3d is not licensed for “personal consumer entertainment”, nudge nudge, wink wink. They’ve even gone so far as to mention it quite prominently on their site (in the FAQ) and the license overview. I’m guessing it must be part of their deal with Microsoft. They do offer an Academic license for $59.95 for student pilots, etc - and this is what most simmers are utilizing. It’s the same as all the other versions, apart from a watermark on the screen while it runs. Apparently a 64bit version is on the cards, but no definite launch date.
X-Plane
After pondering my dilemma with the crashalot FSX and Windows 8, I looked around a bit, and decided to try an extended evaluation of X-Plane. It’s pretty good - runs very nicely, and works well on Windows 8+, and it even has a 64bit version. The only drawback is that it comes on 8 dvd’s and you can only buy a physical copy and have it shipped to you… until now. Just as I was trying to make up my mind … deal with the hassle of having physical disks of X-Plane shipped to me (for $59.99 + shipping), or go with Prepar3d and the watermark. Almost the same price, except for the most probably expensive shipping for X-Plane. And then X-Plane arrived on Steam. Still for $59.99. The download is approximately 3.2gb which includes the initial area KSEA, but has the entire globe as FREE addon DLC, divided in to various areas, so you choose what you want. If you select everything, you’re looking at an additional 53gb installed. Fortunately I managed to copy my existing global scenery, so I didn’t have to foot that entire download 
There’s some very nice free addons - can get photorealistic global scenery, etc, and there are also some pretty decent payware addons, which I will be getting at some stage in the near future.
Anyway, it would be nice to muck around online with some pals some time. I don’t know if I’m too keen to get involved with vatsim (simulator independent) as it may be too rigid.
Anyway, just throwing this out there.
bastards. They come in. Put the others out of business, then mess it up, then leave. Oh well. The gap is back and there are some that will take it. I’ll have a look at X-plane, dust off the old flight sim hardware and see if I can get it going.