Edge 800 devices can accept either TCX (course) or GPX (route) files. Among others, the websites BikeRouteToaster and RideWithGPS let you draw a route on a map (Google or OSM) and then either save a GPX file to your local drive or download directly to a connected Garmin GPS if your web browser has Garmin's Communicator plug-in installed. You don't need an account to use BikeRouteToaster. RideWithGPS requires a (free) account to create new routes.

Similarly, you can install the GoogleMaps CueSheet bookmarklet in your browser, which lets you generate cue sheets and elevation profiles from a route you draw on Google Maps, as well as download the route to a connected Garmin GPS or save as a GPX file.

Garmin's Basecamp app (free from garmin.com), allows you to import GPX and TCX files, and download them to a connected Garmin GPS. Basecamp can be finicky about the format of the files it's willing to import, even if they're syntactically perfectly valid.

I don't know what's available on Windows, but on Mac you can use the freeware apps LoadMyTracks or HoudahGPS to upload/download routes and tracks from/to a Garmin GPS if you don't want to use Basecamp.