You don't really need a dev board for these, you can just stick them in a breadboard and go, they're already in a DIP package and don't really need any extra circuitry besides what you're going to connect them to. You can also program them using basically any other micro-controller you may have, or use a cheap USB programmer to do it, so they're pretty cheap to get started with (Though it's not exactly as user-friendly as plugging an Arduino into USB).

The AT Tiny can be programmed with an Arduino Uno, which is what I used. It's pretty easy. But there are also cheaper boards specifically for the AT Tiny.