Reference implementations of some ITU-T VCEG and ISO/IEC MPEG standards:
H.264/AVC Joint test Model (JM)
H.265/HEVC test Model (HM)
H.266/VVC Versatile Test Model (VTM)
Open access document repositories of the main standardisation groups:
Video Coding Experts Group (VCEG)
Joint Video Team (JVT)
Joint Collaborative Team on Video Coding (JCT-VC)
Joint Video Experts Team (JVET)
Video quality evaluation, metrics and data sets:
Video Quality Expert Group (VQEG)
Research institutes and groups in video coding and telecommunications in general:
Multimedia signal processing group at the Instituto de Telecomunicações (Lisbon site) (link)
Instituto de Telecomunicações (link)
Image and Sound Processing Lab (ISPL) - Politecnico di Milano (link)
NHK Science and Technology Research Laboratories (STRL) (link)
Video coding at BBC Research and Development (link)
Video coding at Fraunhofer HHI (link)
YUV test video sequences:
Xiph.org (link)
Other resources for video coding:
The ryg blog, a very interesting source of information on different topics related to image and video compression (link)
OTTVerse, an OTT e-magazine with plenty of interesting posts and resources (link)
The Turing codec, an open source HEVC software encoder (link)