Are there any standard FPGA internal buses?
Here is a little overview on chip internal buses, which are suitable for FPGAs:


Advanced Microcontroller Bus Architecture (AMBA) from ARM Ltd.
Current version: 5 Specifications
Further reading: Wikipedia

Commonly known buses in that family:


Adavance Peripherial Bus (APB)
Advanced High-performance Bus
Advanced Extensible Interface (AXI)
Variants: AXI-Lite, AXI-Stream

Avalon from Altera now part of Intel
Current version from 12.2015: Avalon Interface Specifications
CoreConnect from IBM
Current version: 4.6 Specification (no official link found)
Further reading: Wikipedia

Commonly known buses from that family:


Processor Local Bus (PLB)
On-chip Peripheral Bus (OPB)

Open Core_Protocol (OCP) from OCP International Partnership Association
Current version:  3.0 Specification
Further reading: Wikipedia
WishBone from OpenCores.org
Current version B4: Specification
Further reading: Wikipedia