They say photography is just painting with lights. So you know what that means... WE BUY ALL THE LIGHTS! There are an endless supply of things you can use to light an image. Things around your house, the sun, a cell phone light, or some tasty fun expensive fancy gizmos and gadgets to help add to your G.A.S. (gear acquisition syndrome). This list is just a needle in a haystack of things you can use.
Godox AD600 - This beast of a light can over power the sun. When I first purchased it, I turned the power to 1/1 (full power) in my living room and proceeded to temporarily blind myself even when I was behind the unit. I've used this as my main head-shot light for a while and loved every second of it. It supports high-speed sync (up to 1/8000 sec), comes with a massive rechargeable lithium ion battery that will last you a full shoot or two easily, and works wireless through a simple and inexpensive transmitter. It's not light, but it's not heavy either. It feels very well built and I've had no issues there. The controls are so easy a child could figure it out. I'd recommend a c-stand or some sandbags to help weigh down the light while in use (especially if you're outside).
Godox AD200 - In the same family as the AD600, but MUCH more manageable and less likely to blind you, comes the AD200. When I'm looking for a simple travel type off camera flash, I bring my AD200 along. It's compact, super reliable, and provides more than enough light almost all the time. It too is wireless and has a rechargeable battery that lasts and lasts. It helps save time with the simple controls and easy setup. You can also purchase two of these and this adapter and get double the light in an easy and compact form.
Godox X-Pro Trigger - If you go the route of Godox lights (which are interchangeable with Flashpoint, too), this little handy trigger is well worth the investment. Before I used this, I would have to test a shot and keep running back and forth to the off camera flashes to adjust. Not anymore. With this trigger, everything you could want is right on top of your camera. It supports up to 5 wireless flashes and I've never had any issues with any of them (even at surprising distances).
Flashpoint Zoom - When I want to shoot with my flash on my camera, I use the Flashpoint Zoom. It's surprisingly compact and bright as I could ever need. With it's lithium ion rechargeable battery, I've never come close to killing a battery on a shoot. The other awesome thing is Flashpoint gear is interchangeable with Godox. So when I utilize multiple flash setups, I can put the Zoom anywhere off camera and talk to it with my X-Pro trigger along side my AD200 and AD600. It's a match made in heaven.
Neewer TT560 - This was my first flash. I currently have two of them and still utilize them on occasion and always have one as a spare flash if anything goes wrong. For less than $35, there is no reason not to own at least one of these. They run on AA's and do the job of any more expensive flash on the market. If you break, loose, or have one stolen, you're less likely to cry! I pair these with a Godox X1R-C which is a receiver it mounts on. This opens the flash up to being controlled with the X-Pro trigger and opens up a whole new world of controlled possibilities!
Lightbulbs - Hell, I've even utilized my colorful Hue lights to add some fun colors to images. Sometimes starting out with constant on (not a flash) lights can help you understand things quicker and see your end results in real time!
Candles - ...maybe the power went out.. Don't play with fire.