Complex sentence: Valve released beta versions of the modding tools on May 10,2011, and supported a competition held by the community mapping website "Thinking with Portals" in May 2011, providing prizes for the most-selected maps.
Split into simpler sentences: Valve released beta versions of the modding tools on May 10,2011. Valve helped to back a contest held by the community mapping site, "Thinking with Portals" in May 2011, providing prizes for the top-selected maps.