Spherical Mercator Projection is used for Google Maps API/Bing Map API.
One of the reasons that the Spherical Mercator projection is so important is that it is the only projection which will allow for overlaying image data on top of commercial layers like Google Maps correctly. When using raster images, in the browser, it is not possible to reproject the images in the same way it might be in a ‘thick’ GIS client. Instead, all images must be in the same projection.
How to create Spherical Mercator projected tiles depends on the software you are using to generate your images. MapServer is a possible route.
Best Answer
OpenLayers has some different popups, but the most similar to google maps, as CaptDragon suggest, is the OpenLayers.Popup.FramedCloud.