Although it runs slowly in the browsers I've tried, this map reveals an amazingly thorough time and space relationship graph for Puget Sound tech companies.
On top of this, LinkedIn has beefed up their company profile pages with illuminating statistics such as incoming and outgoing career paths. Plus, statistics on median age of employees and breakdown of which titles constitute which percentage of company employees sheds light on facts which previously would have required some form of insider knowledge.
Now, for example, I know that prior to joining Microsoft, employees often worked for HP or Oracle, and after leaving tend to arrive at Amazon or Yahoo. I also learned that median age of a Microsoft employee is 34 years old and 13% of its nearly 90,000 employees are software engineers.
Its juicy information like this which I covet for visualization purposes--perhaps simultaneously depicting the flow of workers in and out of a company as well as size and age. I'll have to see if I can snag myself an API partnership.