ModelSim is described as '-Altera Software'. There are seven alternatives to ModelSim for Windows, Linux, Online / Web-based and Mac. The best alternative is gEDA Project, which is both free and Open Source. Other great apps like ModelSim are CircuitLab (Paid), Circuit Tree (Free Personal), Cadence Incisive (Paid) and 5spice (Free Personal).
The gEDA project has produced and continues working on a full GPL'd suite and toolkit of Electronic Design Automation tools. These tools are used for electrical circuit design, schematic capture, simulation, prototyping, and production.
Sketch, simulate, and share your circuits, entirely in your browser -- no install required. Easy schematic capture and powerful SPICE-like, mixed-mode circuit simulation helps EE students, electronics hobbyists, and electrical engineers design faster.