Gerry Marcus responded with a statement which I think nails it: "As e-learning professionals, I believe our job is to get people productively using the tools we've already got."
It's true! We're running around like a boy scout with a 90-blade Swiss Army knife. There's an overflowing toolbox of technologies at all of our fingertips that we could, and possibly should, and probably will be using. That toolbox most certainly includes the four items Curt cited.
But for me, the "most significant and relevant" tool we need to be utilizing as we progress is a strategy, not a technology. A strategy for zeroing in on, defining, and communicating the value of a chosen tool or set of tools, matched to each of our respective learner audiences and the learning objectives they need to master. That's what puts the productive in "productively using the tools"
Brian D. McCarthy reinforces this, saying that "The biggest factor is the ability or desire for corporations to allow and adopt the changes from these advances." We as learning professionals have the opportunity to evoke and nurture that desire on the parts of our companies.
Edwin Stonestreet points out that "Social Networking (In the hands of someone who wants to learn) is an exemplar of what good eLearning can be... However, the elephant in the room is that all of this learning reaches out to that small percentile of people who want it, who seek it out and who are driven to pursue it."
I agree. True of every tool. The tool works if it's matched to the target audience: "people who want it, who seek it out and who are driven to pursue it." The tool works if it's matched to the material and the learning objectives at hand.
So my vote for the most significant e-learning tools? Enlightened selection of the right tools (for the specific learners, and for the specific tasks), and educated and effective promotion for their adoption in our organizations.