Show milliseconds with Android Chronometer

Is it possible to do this?

Not really. You could pass a format string that shows tenths of a second, but the Chronometer itself only updates every second. The update frequency is baked into the code.

If not, is there a free (and preferably open source) library that does the same?

The Chronometer source is available under the Apache License 2.0, so you can modify it to suit. Find all occurrences of 1000 and change them to 100, and you're probably most of the way there.

Bear in mind that this Chronometer can be used in an activity but not an app widget, since customized View classes are not supported by the app widget framework.

Android's default chronometer widget does not support millisecond formatting.

I have modified the Chronometer widget source code in order to show milliseconds. Download it from github.