This is an open-source FPGA project I've been working on for several years now. It's an Ethernet switch for FPGAs, but you can mix-and-match the usual RMII/RGMII/SGMII interfaces with unconventional options like a plain old UART.

My company uses it internally, but we decided to release it as open source. (Currently LGPLv3 but open to other weak-copyleft suggestions.)

Among other things, we've recently incorporated some new technology that allows picosecond-accurate timestamps to be compared across different digital clock domains. You can think of it as a group of NCOs that all track the same best-fit line.