When I first downloaded LinCity-NG (free), I figured it was a simple clone of an older version of SimCity, primarily aimed at the *nix market. As it turns out, it's more like a first cousin once removed, but that's not entirely a bad thing.
If you've played SimCity in the past two decades, you are familiar with the core gameplay elements: You lay out industries, residences, and utilities, you balance economic and educational and social needs, you deal with disasters such as fire, and you try to keep your budget in the black. LinCity-NG, like most freeware games, lacks a bit in the graphics/animation department, but it does offer a number of unique and interesting gameplay elements.
To begin with, tech level is very important. Many items are not available at first, and you must raise the tech level of your city to acquire them. At the beginning, this can be a very slow process, but eventually schools and universities become available. Your city grows from dirt paths to roads and rails, from windmills to coal plants, from simple blacksmith huts to massive industrial complexes. (This is where the relatively low quality of the graphics really hurts, because the variation in images, especially housing, is extremely limited.) Be warned, though--if you are a poor manager, you will start to backslide technologically and lose the ability to place more advanced structures.
In addition to the tech element, various industries require resources, created by placing ore and coal mines. This creates a bit of an RTS element, as you must make sure the output from your mines can reach the industries which need them. Food is also important--farms must be placed to feed your growing population. The game tends to be a bit mute on the subject of missing or unavailable resources; you must figure out for yourself why your mills are silent. (The graphical cues for 'active' and 'inactive' buildings are also somewhat muted, requiring a practiced eye to discern.)
While LinCity-NG does not have the graphical sophistication or bells and whistles of, say, Sim City 4, it does have the advantage of being free, and it runs on lower-end hardware. It also offers a sufficiently different gameplay experience that it's worth getting as a game in its own right and not just as a no-cost knockoff of a commercial game. It is also open source, a boon to programmers looking to see how complex simulations of this type are put together.