First pass at experimental omnibox API. There are plenty of rough edges and

chrome/common/extensions/docs/examples/api/omnibox/background.html

+<script>
+// This event is fired each time the user updates the text in the omnibox,
+// as long as the extension's keyword mode is still active.
+chrome.experimental.omnibox.onInputChanged.addListener(
+  function(text, suggest) {
+    console.log('inputChanged: ' + text);
+    suggest([
+      {content: text + " one", description: "the first one"},
+      {content: text + " number two", description: "the second entry"}
+    ]);
+  });
+
+// This event is fired with the user accepts the input in the omnibox.
+chrome.experimental.omnibox.onInputEntered.addListener(
+  function(text) {
+    console.log('inputEntered: ' + text);
+    alert('You just typed "' + text + '"');
+  });
+</script>