George Bush the Elder referred to it as "The Big Mo", shortly before being mowed over by Ronald Reagan.

Which goes to show that momentum is a tricky force of marketing
physics.

The two big stories in momentum these days are Linux and AMD.  Both are gaining mind share and growing at rates far beyond market averages, their competitors, and kudzu combined. 
Linux is being used in about 1/2 of all government agencies, and 70% want to use more of it.  This mirrors what industry has done with Linux, and mirrors AMD’s chips that now have about 20% of the market share and sales growing at a more-than-healthy 20.5% on a quarter-by-quarter basis.

This bodes well for both, providing they can control their rate of growth.

It sounds like an oxymoronic situation, but market momentum can cause you to lose momentum in the market.  Generally speaking, the market rewards winners.  Companies and products that gain new customers and converts, gain momentum, and as a result gain more customers (repeat until the FTC intervenes).  But growing rapidly creates strains on an organization.  If uncontrolled, these growing pains can cause an organization to fail at some particular function or service, and remove the halo effect of being the market’s favorite child. I believe this was a contributing factor behinds Intel’s ill fated Pentium FPU replacement decision of 1994.

Top management must monitor corporate momentum and assure that the basic brand promises that created said momentum are never compromised.  AMD has gained momentum through listening to customers (creating backwards x86 compatibility for 64-bit systems), providing technology innovation (Hyperchannel), and being price competitive.  AMD appear to be continuing these three functions, and taking growth in proactive and measured steps.

Linux is an interesting study in as much as the Open Source community has redefined momentum by ignoring it.  Certainly, everyone from Richard Stallman to Linus Torvalds wants Linux to rocket past UNIX, Windows, and a few of the minor Saints.  But the goal has never been to dominate the globe.  When I met Linus a few years back, he was rather sheepish about having changed an entire industry, preferring instead to discuss how I/O subsystems could be optimized and how virtualization will aid enterprise server management.  His mindset mirrored that of other Open Source developers I have met.

And therein is where Open Source and Linux market momentum will continue: by staying 100% focused on the core mission, the brand promise.  They want only to create great software that people will want to use.  This all but assures their momentum will continue.

Well, until Microsoft tries to litigate them into oblivion.