Use:
http://gmaps-samples.googlecode.com/svn/trunk/spreadsheetsmapwizard/makecustommap.htm
or:
http://mapmaker.donkeymagic.co.uk/

UPDATE:
These Google Maps tools actually seems better to me:
www.imapbuilder.net – paid, but with some export options free
http://mapalist.com – free

Guides for using Excel with Google Maps:
http://www.ehow.com/how_5879146_create-google-map-excel.html
http://theclosetentrepreneur.com/export-excel-data-to-google-maps