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?!?")
It is not obvious anymore that either product is significantly faster than the other; note that Caldera's Linux offerings have assortedly included versions of both at different times. These products seem to be quite comparable in both price and in functionality. Accelerated X usually supports somewhat more video cards, whilst Metro X provides "multi-headed support" as a no-extra-cost built-in feature (e.g. - support of multiple video cards and monitors on a single computer). Neither difference is likely to be of much importance to the typical user.
Both tend to provide support for new and exotic video hardware more quickly than does XFree86.
A high-performance portable rasterizer library for Type 1 scalable fonts sometimes also called PostScript(tm) fonts.
FontScope allows developers and programmers to build Type 1 scalable font support directly into their applications thus providing a fast, efficient, high quality platform-independent solution to the problem of providing scalable font support.
Build firewall systems using Linux
MPAS - Multi-Platform Access System for file access and resource management.
RMONX provides a web-based interface for a set of SNMP network management tools.
Helius - DirecPC support for Linux Real Soon Now
www.mimestar.com MimeStar - SecurityPro network security monitor
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.
Hylafax is a "free" package that integrates support of faxing, email, and paging. A "unique" thing it does is to provide "gateways" to turn email into any of these.
Various other Unix-based Paging Software
A company called ARDI has created a credible Mac emulator. It essentially emulates MacOS 6.0.5; they have done a clean-room reimplementation which runs under a variety of operating systems, notably Linux, MS-DOS, OS/2, and NeXT.
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