Is it possible to tell Google Maps to use metric units? I know that for instance the German version does use metric units, but I'd rather use the English one.
When you select get directions a link shows up under the input boxes for the addresses for your query that is titled "Show Options". If you select that link you see two check boxes below and on the right of those is a section that says miles / km. You can switch between US standard and metric by selecting the appropriate unit.
As of Aug 2017 you can click the scale bar (in lower right-hand corner of browser-based Google maps) and it will switch from Imperial to metric. Also, MaQleod's answer works for a single session, but now when you get directions you need to click on "Options" (not "Show Options") under the input boxes for the addresses in your query. Neither of these actions work beyond a single session, however, due to how Google determines default units through localization schemes that don't seem to be customizable (yet).
If you use a VPN (e.g. tunnel bear) to fool your browser into thinking you're in a metric country, you can get Google Maps to use metric units in the scale bar by default (in the lower right-hand corner of the browser window). But doing so doesn't seem to change how the units in direction query results.
Whether your browser thinks it's in a country with a metric or Imperial measurement system, Google's localization scheme seems to determine the default ("automatic") units for your directions based on the starting point in your query. For example, a query for directions from Seattle (USA, Imperial units) to Alberta (Canada, metric units) provides distances in miles; but a query from Alberta to Seattle returns distances in kilometers.