nextpnr
Beginner
A free program that decides where each part goes on an FPGA and how to wire it all up.
Novice
An open-source, timing-driven place-and-route tool for real FPGAs. It packs, places and routes a netlist from Yosys and, with an open bitstream database, writes the configuration file. It supports several FPGA families, some still experimental.
Expert
Architecture-neutral P&R in which each architecture implements a device-database API. It has simulated-annealing and HeAP-style analytic placers and a timing-driven A* rip-up-and-reroute router, with backends for iCE40, ECP5, Nexus, Gowin and others.
Explained in From hardware code to bitstream (Architectures).
See also: Yosys, Open-source FPGA toolchain.