According to Phoronix, gaming on Linux has experienced exponential growth in recent times. Over the course of the last two years, Steam's catalog on the platform expanded from 500 games up to over 2200. This is a little over a 4.4x increase over two years. If I'm doing my high-school math correctly, and I seriously hope I am, this corresponds to an average increase of just under 2.1x year-over-year.
In other words, this is litearlly the trend, minus half-life. Snicker snicker snicker.
The quantity of Linux's games catalog is a very different argument from its quality, of course. Still, you can find many interesting titles there. Valve has been porting their catalog to the OS, as have other, high-end titles, like Tomb Raider, Trine, Civilization V, Civilization: Beyond Earth, XCOM, and a couple Borderlands versions. If interested in specifics, and you enjoy a sense of humor like you would see on our PC Perspective Podcast, check out LinuxGameCast for their reviews of specific titles.