Some links to custom markers:
http://www.mapito.net/map-marker-icons.html (flag icons, some of which are used in the example below)
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
http://www.youtube.com/watch?v=Q6GZkZq9DXY)
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.
To save some time & effort you can use one of the "icon KMZ files" (link at top of this page) hosted on this site
and import the custom markers KMZ URL address into your google map's "My icons".
Here's how to do it: http://www.youtube.com/watch?v=g0ru0Zh84YE
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.
The URL's of icons stored on this website were pasted into an excel spreadsheet along with arbitrary latitude/longitude coordinates for each placemark.
The .xls file was then uploaded to the above exceltokml earthpoint.us site, which displayed the icons in google earth.
The KMZ file created with google earth was uploaded to this site (link at top of page) for you to use.
Here is how to use them in Google maps.
Sign into google and go to Maps.
Click on "My Maps". If you have no maps created yet, you will be presented with this page. click on Get Started.
An Untitled map will appear.
Click on "Import"
The following "Import KML" dialog box appears.
Copy and paste the following link into the 2'nd box below:
http://sites.google.com/site/travelmapicon/home/kml-files/markersblueflags1-25.kmz?attredirects=0&d=1
(This and other KMZ links are referenced at the top of this page)
After pasting, occasionally you need to click somewhere outside the box for the "Upload" button to become active.
Click on "Upload from URL" and you will see something like the following.
It automatically renames "Untitled" in the Title box to the name of the file you just imported.
Click on "Done" and then click "My Maps" and you will see the new map under "Created by me"
You now have this group of custom markers when you make your own maps.
The custom icons in this map will be available to use in other maps "custom icons" as long as this map is available.
To show you an example, leave google maps, go to your home page, then back into google maps.
Click on "My Maps" again.
Click on "Create a new map".
Change the name from untitled to "new map for someone"
Then right click somewhere on the map around Kansas and select "add a placemark"
Click on OK, then click on "Done"
You can click on "My Maps" again to see a list of maps created.
Notice that down at the bottom left hand corner "new map for someone" is checked.
Only the standard google map icons are available for use with the new map you just created.
But if you click on the "markers blue flags 1-25" under "Created by me",
it will join the "new map for someone" down in the bottom left corner.
So now you can use the custom blue flag numbered icons with the "new map for someone" and any other map you add under "hide". All maps clicked on under "My Maps" that appear under "Hide" can share icons with the other maps under "Hide".
These custom blue flag markers you imported with "markers blue flag 1-25" can be used with a new map you create anytime in the future just by selecting it from "My Maps". Import other groups of icons and save them to their own map. Select from one or more groups depending on your needs.
The icons contained in these imported maps will appear under "My icons" in your new map.
You can uncheck the box next to "markers blue flag 1-25" under "hide". Those markers no longer show on the active map to the right,
but the custom icons are still available for use.
Now click on "new map for someone" and zoom out until you see the blue marker over Kansas for that map.
Click on "Edit"
Then click on the blue placemark itself
In the dialog box next to "Title" give it a name like "new placemark"
Then click on the blue marker icon to the right of the "Title" dialog box on the map
It will present you with alternate default icons. When you click on "My icons" you will then see all the new numbered flag icons shared from the imported map KMZ map.
Choose an icon from My icons
These and any other custom icons you import with KMZ files will appear in you "My icons" just by clicking on the the imported map icons in My Maps.
Since a video is worth a million words:
http://www.youtube.com/watch?v=oZFzZ42eI0s
--------------------------------------------------------------------------------------------------------------------------------
xls2kml is a VB script for excel that saves to KML
There is also an open office calc file if you prefer.
It offers tons lots of options and comes with cells pre-populated which you can clear to input your own custom icon urls and
generate a bare-bones KML file for google maps using the minimum Name, Latitude, Longitude & Icon url (from this or your site).
Here is an experiment with icon size adjustments and the results in google maps.