Git is a free and open source distributed version control system designed to handle everything from small to very large projects with speed and efficiency.
The downright luxurious Git client for Windows, Mac and Linux. Cross-platform, 100% standalone, and free.
Subversion exists to be universally recognized and adopted as an open-source, centralized version control system characterized by its reliability as a safe haven for valuable data; the simplicity of its model and usage; and its ability to support the needs of a wide variety of users and projects, from individuals to large-scale enterprise operations.
Mercurial is dedicated to speed and efficiency with a sane user interface. It is written in Python. Mercurial's implementation and data structures are designed to be fast. You can generate diffs between revisions, or jump back in time within seconds.
Plastic SCM is a distributed version control designed for big projects.
It excels on branching and merging, graphical user interfaces, and can also deal with large files and even file-locking (great for game devs).
It includes "semantic" features like refactor detection to ease diffing complex refactors.