Free solver for MINP problems

COUENNE

An open-source deterministic global MINLP solver.

BONMIN

An open-source local MINLP solver.

Octeract Engine

(our own solver) is a commercial massively parallel deterministic global MINLP solver that is free for academics. What's more interesting is that starting next month the 1-core version of Octeract Engine will also be free for commercial purposes (therefore for everyone). We're still setting up the web framework for people to get that version automatically, but in the meantime you can just request a commercial trial on our website and we'll send you the free 1-core licence automatically when the trial runs out.

Since I have written a lot of the code I cannot overstate how biased I am, but this is the solver I would recommend - as of the current version (1.9) the 1-core version finds feasible solutions for 94.5% of 3,016 test problems (pretty much every test problem ever published) within 30 minutes, which is vastly superior to open-source alternatives.