Dota 2 [official site] has jumped to Valve's shiny new Source 2 engine with the open beta release of the Reborn update. The main version of the game will still run on the old engine until after this year's International tournament, but everyone can now poke at the new one.
If you dig Dota 2, hey, go have a poke at what's to come; your match history, matchmaking rating, cosmetic items, and so on are shared across both versions, unlike normal Dota 2 test clients. If you just want to know more about Valve's new tech, the engine that'll be free(ish) for everyone to make games on, read on.
Valve have a hefty post explaining the tech coming to Dota 2 with Reborn. You'll find reduced input latency, better performance, a unified lighting system, better use of multicore CPUs, an improved audio that they say noticeably improves voice quality and latency among other benefits, new physics and cloth simulation, native OpenGL support that'll make Mac and Linux versions faster (Reborn's Mac and Linux clients are due "in the coming weeks"), and streaming loading that lets animation, voice chat, and so on continue while loading. It also brings a new version of Source Filmmaker, with new features like editing replays. Go read!
The rebuilt version of the Dota 2 map has a few little changes. Check this comparison gallery put together by 'Gollum999'.
Valve are also reworking Dota 2's networking, with a new network backbone and improved routing that should make for faster connections. It should be more resilient against DDoS attacks too.
Also arriving in the Reborn update are solid support for Custom Games (mods, basically), and a new UI, as Valve detailed over the past week.
You can check Dota 2 Reborn out now by firing up Dota 2, going to the Play tab, clicking that Dota 2 Reborn Beta button, then waiting for the 7.1GB download to finish.