The icons in the 2 sets below can be used for free under the terms of the Creative Commons Attribution licence. In short, that means you can use them for almost any purpose, but a fully functional link to the source (this page, http://www.mapito.net/map-marker-icons.html) is required, either on the page where you use the icons or in the copyright section of your website.
Click on a set of icons to download a zip file (150KB) with all the icons shown in the set.
These icons have been created for use on maps with a legend in a sidebar when more than 26 id's are needed. For up to 26 id's you are better off with capital letters (A to Z) on the marker: they need only one character, which allows the markers to be a bit smaller. And each pixel counts if you don't want your whole map to be covered with icons.
Examples of the icons in action can be seen on these web pages: