Limitless/WMG

Eddie was still on NZT-48 at the end of the movie.

The whole being off of NZT-48 but still keeping most of the brain processing power with it thing, was Eddie lying to Carl to avoid his threats. After all, NZT-48 does make you feel "invincible", and it is not approved and illegal. He would also probably be lying to Lindy as well.

This is supported by the fact the he offhandedly states that he has more than one lab. Why bother bringing that up, if he was off the stuff anyway?
He also says that he "tweaked" it. He's not on NZT, he's on something similar that the labs managed to cook up for him that isn't quite as dangerous. (And NZT was never actually illegal, it just doesn't exist as far as any regulatory agency is concerned - you can't get a prescription for it, but the DEA isn't going to come busting down your door if you have a supply. In the real world, there are lots of rejected or experimental drugs that you can buy from chemical supply companies as "research chemicals" without attracting too much attention.)

NZT was actually just a really small amount of meth, modified to be ingested orally.

Pretty self-explanatory. Eddie was addicted to meth and only felt "limitless" when on it. He already had the ability to become an investment genius etc. It just made him extremely focused and organized. This also fits with the fact that the 90% of Your Brain theory is just a myth.

NZT is used by the Assassins of Assassin's Creed.

The real life Assassins used Hashish (aka cannabis) so they already have a taste for mind-altering drugs. Add the fact that NZT makes you like jumping off high places and better at fighting multiple enemies, and it seems obvious that NZT would their drug of choice. It also meshes well with the Assassin's philosophy: that people are free to risk their own mistakes in an attempt to become better people.

Eagle Sense also seems to be a way of representing NZT's heightened insight.