I use these things A LOT! They are great for temporary builds, as you can very easily disconnect and reconnect wires using them. They are also certified for permanent installation. Best of all- they will connect everything from 20AWG “alarm” or “network” cable up to 10AWG, mix and match, stranded and solid! They are good for low voltage as well as 120v.
Update: There are actually two types:
221-41x – 22-12AWG
221-61x – 20-10AWG
Most home centers have the 41x (22-12AWG) version, which is also the best one to use for holiday light and general home applications.
They are not water/weather resistant, so they do need to be used in some kind of enclosure for outdoor work, but they are a great alternative to soldering, crimping, or old-school wire nuts.
WAGO recently re-designed the Lever Nuts. They used to be a bit blockier with smaller orange levers. I used those previously and loved them. You will see them on some of my older builds, or re-used for test builds.
You can buy them at almost any hardware store, home center, electrical supplier, or of course through eBay and Amazon. In fact- since they were re-designed, the older style ones can be had very cheaply in huge quantities as of this post.
There are Chinese-made rip-offs, of course. I’ve used some of them for low-voltage applications and car audio stuff. They tend to have smaller levers that are harder to work with and use flimsier plastic bodies. They may work as good as a WAGO, at least for a while, but I don’t personally trust them.