1to4 is/was/whatever a spiritual successor to Heartbeat. To use it is to have a conversation with an earnestly interested party about how you're doing, using simple, unambiguous terms.
I wrote a letter to the user, published at /why, to set the tone for its use. Though, of course, every piece of it was designed to give off the same vibe of simple love, no matter how much of it the user actually found. And parts of it are meant to be "found", through exploration, just like a conversation. It operated almost like a piece of interactive fiction, but one in which the user is writing the story.
1to4 texts the user daily to see how they're doing on a 1-2-3-4 scale, and then sends that number to any configured recipients -- anyone who the user trusts to understand what a "4" or a "2" (or whatever) means for them. The recipient can text back a heart. That's it. I designed it to be a low-emotional-cost method for staying connected with others.
It used a passwordless auth system, texting the user a numeric code to enter back into the interface for authentication. This solidly predated the proliferation of two-factor auth, which is neat. :)