Lattice has been by far the favorite of the FOSS community, but there's been more news:
- https://github.com/YosysHQ/apicula has appeared for Gowin FPGAs found on e.g. Sipeed Tang Nano boards (very cheap on AliExpress) - a vendor called QuickLogic made SoCs that only use the FOSS toolchain for the FPGA part, out of the box: https://www.quicklogic.com/products/soc/eos-s3-microcontroll...
i'm interested in the OSS flows but i haven't dug in yet. so some questions (if you have experience): isn't it only for their ice40 chips? and how smooth is the flow from RTL to bitstream to deploy?
one hesitation i have with jumping in is that i'm working on accelerator type stuff, so my designs typically need on the other of 30k-50k LUTs. will yosys+nextpnr let me deploy such a design to some chip?
Icestorm is for iCE40, Trellis is for ECP5 (which comes in variants up to 85k LUTs);
the flow is simple enough to do manually but there are things that make it one-click. This tutorial series https://youtube.com/playlist?list=PLEBQazB0HUyT1WmMONxRZn9Nm... uses one.
As for handling really big designs, I don't know.