What you should have on hand at all times just incase something were to happen to your chickens.
Poultry Care Spray- This is good for pecking wounds, bumble foot, vent prolapse, etc.
Corid- Used to treat Coccidiosis, an intestinal disease caused by parasites.
(For more info on Coccidiosis Click here!)
Rooster Booster Poultry Cell- A good mix of vitamins used for chickens that seem to be under stress.
(when introducing new chickens into the flock, bullying for other chickens, etc.)
Oyster Shells- Chickens need calcium to make sure their egg shells are hard. If your chicken is laying soft shell eggs they might be deficient in calcium. Adding oyster shells to their food can help get their shells hard again. This should be given regularly if your chicken food doesn't already include it in the mix. 
Vet Tape- This is good to have if your chicken gets injured and the wound needs to be covered to prevent infection. 
Epsom Salt- Used to treat bumble foot and egg bound chickens.
Mix in warm water and let them soak in it for a couple minutes.
Bag Balm- Used for wounds and scaly leg mites.
Preparation H- Works well for prolapse vents.
Cornstarch- To stop bleeding.
Gauze Pads- For packing wounds.
Scissors- A designated animal use pair of scissors for cutting gauze or bandages.
Apple Cider Vinegar- Used as a preventative for illnesses.
1 Tbsp per gallon of water once a week.
Electrolytes- Whether you are getting new chickens, a hot summer day, or an ill chicken it’s always a good idea to have them on hand.  I prefer to buy single dose packages or single-dose pills.  One packet or pill to each gallon of water.
Probiotics- Also available in single-dose packets I always have them available.  At the first signs of stress or illness, I add them to the water for a day or two.
Ivermectin Pour-on- I use this for a few different reasons. It works great for intestinal parasites as well as external.  Though it has not been tested in chickens it has been used by many chicken keepers for years.  The dose I have been told is large rooster 5 drops, large hens 4 drops, medium chickens 3 drops, bantams 1-2 drops, and never use on chicks. I use a small eyedropper to administer the correct dose to the skin between the shoulder blades. As far as egg withdrawal, I’m not 100% sure because there is no research; I would recommend no less than 30 days.
Syringe- To administer medication or to use as a wound flush.
Disposable Gloves- Dealing with chickens can get messy and you're not going to want to touch the gross stuff. 
Lubricant Jelly- To help with egg bound chickens and prolasped vents.
(Yes you will be sticking your fingers up the vent to help get the egg out.)
A "Hospital"- We use an old dog crate as our hospital when they have to come into the house to be watched over and treated. You can also use a plastic storage bin.