In terms of Add ons there are hundreds,
I don't believe these companies are classed as competitors so I think I can name them for you to check out as they have many add ons for whatever you fancy, also there are numerous freeware places to get add ons (best to just google).
Aerosoft
Just Flight
Sim Market
As for traffic, the best products for this are
Traffic X from Just Flight and
MyTraffic from Sim Market
These add a huge amount of detail to the flights into and out of airports and can be based on real time flight details.
As for other add-ons I'm a bit of a junky on this so have loads so have listed some below.
Horizon VFR Real Scenery ( puts a new layer onto the UK maps to provide real photographic imagery, so you can fly over your house and see it)
HeathrowX - Updates the Heathrow area to allow for the current (inc Terminal 5) layout of heathrow.
Real Environment Extreme (REX2) - Allows modelling of the real time weather and updates graphics for many items e.g. runways/clouds.
Active Sky
Active X
Numerous new planes/helicopters
A number of other airport enhancements, JFK, Chicago, Lukla etc.
I've probably spent in the region of £250 on add-ons for FSX, your possibilities are endless depending on what you want to do with it and how much processing power you have in your PC.
Hope that helps a bit.
Taff