How can I display a map with multiple markers?

You can't add multiple markers on the OpenStreetMap.org homepage. For point & click solutions you might like geocommmons, StaticMap, and others, but for more power and flexibility it's worth learning a few javascript tricks:

Check out: this multiple markers OpenLayers example

Do "view source" to see how it works, and feel free to copy it. Some things to note:

• It uses the open source JavaScript library called OpenLayers (other 'slippy map' libraries are available)
• The map is initialised with a basemap layer from the default OpenStreetMap tile server (other tileservers and alternative renderings of OpenStreetMap data are available)
• It adds a 'Vector' layer, where the markers are defined.
• It needs a marker image file placed in a 'img' folder alongside
• It adds three markers. If you were adding more, you might want to do this by defining an array first, and then looping through this to call vectorLayer.addFeatures
• As an exampe of event handling, it detects the event of selecting the marker, and calls 'createPopup'

This is not yet possible for the slippy map on the front page. But you can check out OpenLayers which is an open source javascript slippy map that you can deploy on your own site.

The short answer is that we don't provide a mechanism to do that directly on the web site, but that doing so isn't hard at all if you have a little web development experience (which it seems you do). All you need to do is use OpenLayers and use OSM as your background layer.

Here's a URL of a talk I prepared for State of the Map US 2010 (but never got to present), which will take you through the basics of web development using OSM:

http://www.emacsen.net/osm/osm-web-tutorial.pdf

If you go through that presentation, you should hopefully have a good foundation on how to use OSM on your site.

Leaving aside the "ajax" bit, you may find that http://open.mapquest.com/ (or other country site) does what you want - it uses OSM data, has a slightly different rendering to OSM's mapnik, but does have a rudimentary "stick a marker here" logic.