http://www.bibliotecapleyades.net/esp_einstein.htm This site makes such claims including:
Jules Henri Poincaré (1854 - 1912) was a great scientist who made a significant contribution to special relativity theory. The Internet Encyclopedia of Philosophy website says that Poincaré: "sketched a preliminary version of the special theory of relativity" "stated that the velocity of light is a limit velocity" (in his 1904 paper from the Bull. of Sci. Math. 28, Poincaré indicated "a whole new mechanics, where the inertia increasing with the velocity of light would become a limit and not be exceeded") suggested that "mass depends on speed" ("formulated the principle of relativity, according to which no mechanical or electromagnetic experiment can discriminate between a state of uniform motion and a state of rest" "derived the Lorentz transformation" It is evident how deeply involved with special relativity Poincaré was.
Even Keswani (1965) was prompted to say that, "As far back as 1895, Poincaré, the innovator, had conjectured that it is impossible to detect absolute motion", and that "In 1900, he introduced 'the principle of relative motion' which he later called by the equivalent terms 'the law of relativity' and 'the principle of relativity' in his book, Science and Hypothesis, published in 1902". Einstein acknowledged none of this preceding theoretical work when he wrote his unreferenced 1905 paper.
In addition to having sketched the preliminary version of relativity, Poincaré provided a critical part of the whole concept - namely, his treatment of local time. He also originated the idea of clock synchronization, which is critical to special relativity.
Are these claims incorrect? I already know Hilbert was wrong. Edit: more info from quora: http://www.quora.com/Did-Einstein-plagiarize-Henri-Poincar%C3%A9?no_redirect=1