The whole envelope looks great with the 25 1c stamps, but the main feature is the one celebrating Jurgis Mačiūnas (anglicized to George Maciunas), one of the founders of the Fluxus art movement.
There are various fonts on the stamp itself, as well as on the selvage. Here's the full sheet - even the layout of the stamps is quirky.
For more interesting fonts on postage, check out the links at Sunday Stamps.
And some really great things were inside the envelope, too, including a very clever approach to make artistamps by incorporating selvage from a real stamp sheet.
Mindaugas also included these great postmarks - I love the one with the mouse.