Google maps does not offer many default icons.
(at least at the time of this writing, November 2009)
Numbered or alphabet icons help differentiate locations.
You can add the blue letter markers below individually to your "My icons" in google maps.
select "add an icon" and paste the above URL into your google map
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You can also import groups of custom markers into google maps "My icons" with KMZ/KML files.
This page describes how to do it.
This video does the same.
You may want more markers or have control over where your custom "My icons" are hosted.
This site http://www.earthpoint.us/ExcelToKml.aspx
helps when creating your own KMZ files with multiple custom markers to upload to your website.
This video describes how I used the above ExcelToKML website to import custom markers.
Some links to custom markers:
http://www.mapito.net/map-marker-icons.html
http://mapki.com/index.php?title=Icon_Image_Sets
http://code.google.com/p/google-maps-icons/downloads/list
http://www.benjaminkeen.com/?p=105
http://www.mapchannels.com/Markers.aspx
http://sites.google.com/site/gmapsdevelopment/
http://groups.google.com/group/google-maps-api/web/examples-tutorials-custom-icons-for-markers
http://www.powerhut.co.uk/googlemaps/custom_markers.php
http://www.lumiere-couleur.com/pub/soft/markers
(use website listed above to create your own custom markers)
Unzip markers, upload them to a website (create your own free googlesite) and link to the URL for custom markers in your own maps.
To put custom icons in your google maps "My icons", they need to be stored on a server somewhere, whether on your own website, google's, flicker, etc. Or feel free to link to the custom markers here.