I'm familiar with COIN-OR and have also used a couple packages in R to solve LPs. Today I found out Google has their own open source optimization software, and it got me wondering what other open source LP solvers are out there?
OpenSolver is an LP/IP/NLP solver that plugs into Microsoft Excel. I used it for some classroom stuff a while back and was quite pleased with it.
If you are interested in metaheuristics, there are quite a few open-source contributions floating around (about which I mostly know nothing). I have used the Watchmaker Framework for Evolutionary Computation (i.e., genetic algorithms) successfully in a couple of projects. I don't think it is under active development any more (although the developer might respond to pull requests), but it is quite well documented and works. Not long ago, I looked around and found some other Java-based GA libraries, but have not gotten around to testing any.
I compiled a list of solvers I could find last year. Several are COIN-OR-affiliated, but others include Mini-CP, DSP, BiqBin, OSQP, ECOS, and Dakota. (Edit - not all are dedicated LP, see comments below)
Just to add another, easily google-able, resource: Wikipedia, more or less, "maintains" a "List of optimization software" -- which includes the super handy "Mathematical optimization software" template I -- disclaimer: shameless self-plug -- started 6 years ago (and never bothered to curate) [the latter also includes references to alternat(iv)e taxonomies in its "talk" section].
Description (from Wikipedia):
The GNU Linear Programming Kit is a software package intended for solving large-scale linear programming, mixed integer programming, and other related problems. It is a set of routines written in ANSI C and organized in the form of a callable library.