"František Kupka (September 23, 1871 - June 24, 1957) was a Czech painter and graphic artist. He was a pioneer and co-founder of the early phases of the abstract art movement and Orphic cubism (Orphism). Kupka's abstract works arose from a base of realism, but later evolved into pure abstract art." View Wikipedia's article.
It's interesting to see the range of his work, from paintings depicting the gardens of Babylon, to portraits, to completely abstract paintings. I always find artists whose work constantly evolves and changes to be the most inspiring.