It doesn't involve the modification of any of the game files or code, so A) it doesn't break the game in any way, and B) Doesn't break the TOS (for any other MMO out there - I do not know FCs policy, and if this thread is taken down, I will not argue)
One such piece of 3rd party software, which many of you may be familiar with, is called SweetFX - to put it simply, it's basically a piece of code that sits between the game and your GPU that adds in extra effects and fun little toys. With the new engine, not only have (many of us) gotten a better performing game, and more built-in shiny effects ( guys? FXAA?), but with the move from DX7 to DX9, we also have the ability to use 3rd party software to make the game assets look a whole lot better - and lets face it, Michi and the crew don't have the time or funding to remake all of the game assets, so really, why not do what we can on our end? Posting this here because I can embed screenies.