KML4Earth

Reference

KML Circle Generator generates a circle at center of current view or provided center point

If auto refresh checked then circle will be redrawn whenever user stops otherwise circle is drawn when Network Link is initially loaded and when it is manually refreshed.
Circle radius: Auto refresh
Center latitude: / longitude: [decimal degs] (optional)
Circle color:

Note the circle geometry is clamped to ground so it follows the terrain.
A hilly or mountainous region will show distortions of the lines as it clamps to the ground.
You Are Here demo - uses NetworkLink to update with changes to the view.
If you click the placemark it will show other view information available from Google Earth including bounding box coordinates and view tilt angle.
When you stop moving a Placemark will be dynamically placed at the center of the view using the position provided by Google Earth.
numbers (NEW) -- Generate numbered point placemarks for all coordinates of each LineString, LinearRing and Polygon
centerPoint (NEW) -- Generate center point for non-point Placemark geometries including GroundOverlays
bbox (NEW) -- Generate bounding boxes for KML with non-point Features including GroundOverlays and Regions
Other KML tools can be found on the home page (e.g. Big KML Generator).

http://www.kmlvalidator.com/home.htm	Allows file upload
http://www.feedvalidator.org/	Validates Atom, RSS and KML
http://www.nearby.org.uk/google.html	Handful of useful Google Maps and Google Earth tools created by Barry Hunter (another Google Earth Forum top contributor)

libkml -- Google's library for use with applications that want to parse, generate and operate on KML.
Written in C++ with bindings available via SWIG to Java and Python.
JAK: Java API for KML -- Java interface for easy access to KML data

Simplekml -- Python KML library

Following tools support XML (and non-XML) sources including KML files:

Google Earth (TM) and Google Maps (TM) are registered trademarks of Google Inc. KML4Earth is not affiliated with Google.