I just noticed some icons on the right side of the url in Chrome (on a Mac) with favicons of technologies used by the current website.
Looks like the work done by the "AppSpector" extension (also see the extension author's page about AppSpector).
What are these icons called?
The icons in this scenario have no specific name, they are just icons added by the extension to represent the technologies it finds.
how does Chrome know what the site is using?
The extension detects what's used by checking for common libraries required for the various web/tech stacks, as well as checking header information to determine web server make, etc.
This extension will help web developer to inspect web framework / CMS and javascript library running on current browsing website. An icon will appear on address bar indicates the detected framework. Version detecting is being implemented.
Currently, this extension can detect more than 100 popular CMS and javascript libraries, and more will be added in future releases. Visit extension website for more detail.
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It would also be nice to know if they can be modified, suppressed, etc.
This is a client-side detection, the only way you could prevent it on machine that are not yours (as a website creator), is to not use libraries it recognizes.