The first is a full-featured proxy API, which will, for example, allow users to set different proxy servers for normal browsing and Incognito mode. Proxy auto-config scripts are also supported by the API.
The second -- Web Navigation Extension -- is a bit more expansive. This API will allow devs to build everything from more powerful safe browsing extensions -- like Traffic Light -- to data analysis and reporting extensions.
Both APIs are currently experimental, so you'll need to enable them on the about:flags page to try out any relevant extensions. Apart from a proxy example built by Google and shipped with the Chromium source, we're not aware of any examples just yet, however. We'll let you know when we spot any slick, new extensions which do surface.