The most notable freely available set of X servers are known as XFree86, which is included with all of the Linux distributions.
The main reasons for choosing a commercial X server over XFree86 are fourfold:
The commercial servers support video cards not yet supported by XFree86. They support some video cards that cannot be supported by XFree86 since writing drivers requires documentation that is only available under nondisclosure agreements;
They've been tuned to provide better performance, or provide support for "acceleration" features your video card has that XFree86 does not support; (Or so one hopes. Hard numbers/specs on this seem to be hard to find.)
The X servers provide configuration utilities to make them easier to configure than XFree86;
The XF86Config file is rather daunting. Mind you, it tends to provide additional functionality in terms of getting that last iota of resolution out of your video card and monitor, but it does have a steep learning curve. XFree86 configuration utilities have improved lately, so that this is not as much of a benefit as it used to be.
They also allow you to readily link in additional X services that may not be available for XFree86 such as an official implementation of OpenGL . ( Mesa is a "libre" system available for various platforms including Linux that bears amazing resemblance to OpenGL, and runs a large proportion of OpenGL apps; it, however, cannot claim to be an OpenGL implementation...)
The "fast" X. I have used Accelerated X to drive my Diamond Viper card. Their old URL was www.xinside.com; apparently they have changed the company name because they were getting monstrous numbers of web "hits" from people looking for sexually-oriented materials. ("Shouldn't X-Inside mean that you've got really 'hot stuff' inside?!?")
GCOM, Inc. has announced their Linux STREAMS X.25, SNA, Frame Relay, SDLC, HDLC, LAPB, LAPD, QLLC and BiSync Data Communications Protocol Suites. The GCOM Protocol Suites add both software and hardware to provide a synchronous capability to a PC platform running Linux.