Make sure you pick up a copy of the newest Fluxx Promo Card, "One, Two, Five!" The text on the card says "Anytime the numeral 3 appears on another card, treat that number as though it were really a 5." It can be used in any version of Fluxx, though if you put it in a deck that includes the card Inflation, we recommend you discard Inflation when 1,2,5 goes in play, and discard 1,2,5 when Inflation goes in play, unless you want to work out some VERY creative house rulings for how these two cards would work together.
Also check out these Fluxx Promo Cards: "Start the Clock", "Zombies Eat Brains", and "Larry". These promo cards were made to celebrate the launch of Zombie Fluxx. The "Start the Clock" and "Larry" promo cards can be used in any version of Fluxx, and "Zombies Eat Brains" works with Original Fluxx (you need "Death" and "The Brain" to win with this goal). They come on postcards that have a punch-out promo card (we recommend cutting the promo card out with scissors for a slightly cleaner edge).
Here are all the other promo cards that are available.