Currently using an I2C 16x2 display which is overkill and too big, going to switch to 7 segment LED x 2 I think plus shift register(s).
Currently using a mini pot for temperature set, may switch to +/- buttons.
Currently have no relay connected, just a piezo beeper when water reaches correct temp.
First used High-low tech method for programming ATtiny, but switched to arduino-tiny cores in order to gain tone() support for my piezo. I think the arduino-tiny cores seem to work a little better anyway, although it's SLIGHTLY more work to install them.
I'm using an Arduino UNO to program ATtinys right now using a custom "shield" (some pins and wires soldered to a socket and a glob of hot glue) according to the diagrams at high-low tech.
Currently powered by rechargeable USB li-po battery/power supply... going to include a 5v switching power supply from a half-broken phone charger hidden in the case.
Ideally, in the end, I will plug this unit into the wall, plug the kettle into this, and dip the thermometer into the water. Everything will be contained in as small a package as possible. Probably will cost more than a proper temp-controlled kettle but I travel a bunch and drink tea that needs the water to be ~75*C. Plus this is a fun way to learn to push the ATtiny for small embedded projects.