Ich sitze mit einem Philosophen im Garten; er sagt zu wiederholen Malen "Ich weiß, daß das ein Baum ist", wobei er auf einen Baum in unsrer Nähe zeigt. Ein Dritter kommt daher und hört das, und ich sage ihm: "Dieser Mensch ist nicht verrückt: wir philosophieren nur."
(I sit with a philosopher in a garden; he says repeatedly, "I know that is a tree", while pointing to a nearby tree. Someone else comes by and hears this, and I say to him, "This man isn't mad; we're only doing philosophy".)
-- Wittgenstein, On Certainty
To say that Wittgenstein left no lasting impression on philosophical thought is doing him a vast disservice. To be sure, he repudiated in the Philosophical Investigations his early claims from the Tractatus. Likewise, he never really had a school of followers (unless one counts the logical positivists and the ordinary-language philosophers, all of whom Wittgenstein considered to be misinterpreting him). However, the influence of the language-based philosophy of the Investigations can still be felt, in philosophy as well as literary criticism. Much as the Renaissance and Enlightenment saw a movement from society to the individual as the fundamental philosophical entity, the twentieth century saw a movement away from the study of the individual to a study of language; Wittgenstein's philosophy (though not the man himself) has been one of the leading driving forces behind this change. Reports of the death of Wittgenstein's ideas have been greatly exaggerated.
If you liked Philsophical Investigations, you may also enjoy On Certainty (Über Gewissheit); this work, collected from notebooks Wittgenstein kept over the last few years of his life (the last note was written two days before his death), is in large part a response to G.E. Moore's "In Defense of Common Sense" and "A Refutation of Idealism". It deals with the nature of knowledge and certainty, especially as grammatical concepts (in the Wittgensteinian sense of "grammatical").