 facto standard driven primarily by a single vendor: Intel. Microsoft Windows is also in the running, but it was very arguably x86's ubiquity in a segment of the market open to relatively low-cost packaged software that made the rise of Windows possible. Over the past decade, AMD has also driven x86 innovations--most notably 64-bit extensions. However, it was Intel that had the biggest hand in shifting the industry from a structure in which each company did everything from fabricating processors to writing operating systems to developing databases to one in which different companies tend to specialize in one part of the technology ecosystem.
x86 emerged as a dominant chip architecture for a variety of reasons. IBM designed Intel's 8088 into the first important business PC. It got this win and others at a time when the world was rapidly computerizing. And Intel optimized itself to ride key technology trends while divesting itself of businesses, such as memory, as they commoditized.
Finally, here are a few others that could make a list like this one:
Wi-Fi played a big role in making personal computers more mobile--which is why Intel pushed it so hard.
VGA is the computing video standard that finally helped merge a rather splintered landscape and had a good long reign. (The latest video interconnect trend is a shift to HDMI--representing a coming together of computing and consumer electronics standards.)
SCSI was the first storage interconnect to merge in a big way a disparate set of existing connection schemes, both proprietary and more or less standardized. However, storage has remained an area where different standards are used for different purposes. That's changing to a degree with SATA, however, which we now see in both PCs and data centers.