H.265 (aka HEVC) video codec will eventually supplement H.264 over the years as it offers better compression at the same bitrate, or better quality for the same compression ratio. There’s a already some experimental implementation that allows to (slowly) decode and encode H.265 on your PC, as well as some HEVC implementations optimized for ARM SoCs. At least two other codecs aim to compete against, and better, HEVC: Google VP9, and Daala codec (temporary codename) developed by Xiph.org, the open source community that brought Vorbis or Theora open source codecs to the world.
The Xiph developers explains that next generation codecs such as VP9 and HEVC are incremental improvement of codec designs using DCT (Discrete Cosine Transform) that have been used for years, and they’ve decided to take a new approach with lapped transforms, a technique that is also used in audio codecs such as MP3 and Vorbis. Lots of patents claims for video codecs are related to DCT, so using another method may also avoid legal issues.
The algorithm used by Daala is based on 3 different IEEE publications, and is partly described on a “demo” page on Xiph.org. Developing a new video codec is far from trivial, so I assume it may take several years before a usable implementation is available. Source code is available on xiph git repo, and further information is available on Daala page.