The most important part of any custom mechanical keyboard is, arguably, the switches. They come in various weights, designs, and feedback types, all of which can completely change the typing or gaming experience. There's one name in particular you'll hear mechanical keyboard enthusiasts throw around a lot: Zealios. The Zealio is a custom Cherry-style tactile switch that has earned legions of fans despite a high price tag. For many keyboard fans, it's the "endgame" switch, but what makes Zealios so special?
Zealios are tactile switches, which means they don't produce a click when you press them. However, there's a bump you can feel just above the actuation point. They were designed by ZealPC and manufactured by Gateron, which is a maker of Cherry "clone" switches. After Cherry's patents expired in 2012, Gateron started making switches that replicate the Cherry look and feel at a lower price. Zealios aren't cheaper, though.
The original idea with the Zealio was to imitate the Ergo Clear, a sort of "Frankenswitch" that people have been making for years by putting lighter springs in a Cherry MX Clear and adding lubricant to the stem. So, the Zealio has a big tactile bump and comes in few different spring weights, most of them heavier than mainstream switches like browns and blues.
You can identify Zealios by their vibrant purple stem and transparent housing. Otherwise, they look very much like a Cherry switch. They work with Cherry-style keycaps and PCBs as well. The switches come in 62g, 65g, 67g, and 78g weights. The 62g is like a heavier MX brown with a larger bump, and the 78g is probably the closest to an Ergo Clear. While you can add lubricant to Zealios by opening up the housing, they're already impressively smooth. Gateron's tooling allows for a very uniform texture on the slider, whereas Cherry switches are slightly rougher.
Zealios are like the Cadillac of keyboard switches, and you'll pay handsomely for them. ZealPC has the switches produced in batches every few months, then sells through stock before starting a pre-order for more. It's currently up to round nine with a pre-order price of $0.85 per switch. After the pre-order, the price goes up to $1 per switch. You're looking at $50-60 just for switches if you want to build a small 60% keyboard with Zealios. Other switches might cost half as much.
Are Zealios worth the price? That depends who you ask, but I've used them in several custom builds and I intend to continue using them.