Google launched My Maps, a new feature in Google Maps that lets you mark some places on the map and annotate them. Essentially, it's an easy way to create a KML file from Google Maps (until now, the feature was available in Google Earth). To create a map:
* Select one or more places from the map.
* Draw lines and polygons to highlight paths and areas.
* Add descriptions by typing HTML code or using a rich text editor.
* More in this user guide.
Each map will have its own permalink (like this map of Googleplex) and can be exported as KML to see it in Google Earth. Google wants to create a big directory of maps, so if you decide to make your map public, anyone can find it in the search results. The alternative is to make it unlisted, which means the address won't be included in search results, but it will be accessible to anyone who knows it (much like in Picasa Web).
You can find maps created by other people if you search for a local business and click on "see user-created content" at the bottom of the search results. This acts as a search engine for KML files available on the web, so you'll find a lot of KML files from bbs.keyhole.com (Google Earth Community), where you could host your KMLs and get some feedback.
This new feature (available for a long time in Windows Live Maps or in some Google Maps mashups) is also a good way to share more than one place with a single link. You can add placemarks manually or from the search results.