If you play 5e D&D I can recommend Crit Games Combat and Condition Cards enough. I picked them up at Origins this year and they are super useful. They are really nice double-sided card that make it easier for a DM to keep things flowing.
The Condition Cards contain four of each of the following cards, Restrained, Blinded, Charmed, Deafened, Frightened, Grappled, Incapacitated, Invisible, Paralyzed, Petrified, Poisoned, Stunned, and Unconscious. Conditions are a big part of 5e combat and this is an easy way for your players to keep track of them.
The Combat Cards have six number initiative order cards, six Death Save Success/Failure cards, and four of each of the following card: Dying, Concentration, Exhaustion, Mounted, Prone, Underwater, Cover, Incorporeal, Hidden, and Flanking. With these cards you you and your players have easy access to common combat rules that while common (aren't easily memorized).
These are bonus cards from the Kickstarter that funded the creation of the cards. The initiative cards work with the ones from the Combat Cards and takes it up to tweve. The are four Inspiration and four Intoxicated cards. Intoxicated is a condition that was in the D&D Next playtest but didn't make it into the final version. They are rough, but I like them. For alternate take, you can check out my 5E intoxicated rules.