I recommend Etranger with reservations. Provided you keep its limitations in mind, it's a great site.http://www.users.globalnet.co.uk/~dheb/2300/index.htm
Some of the resources on there are good. Sadly, most of it is very "Flintstones" - the world militaries circa 199X-200X with gosh wow future gun tech and a sprinkling of hovercraft and combat walkers. However, their tactics and other equipment besides guns are all still modern day. You'll see a lot of lists of TO&E but not much daring thought as to implications of advancing technology. In a lot of ways, that vibes with 2300, which tends to be pretty low on the consideration of implications as well, but I think a lot of the articles on that site go even further and are pretty much Twilight: 2000 with zap guns.
There are, however a few really good articles on there and one of the CG artists on there is top notch.