A sample: custom title source of Silverlight Bing map

In Silverlight web page:

<m:Map>
    <m:Map.Mode>
        <mCore:MercatorMode/>
    </m:Map.Mode>
    <m:Map.Children>
        <m:MapTileLayer>
                <m:MapTileLayer.TileSources>
                        <local:CustomTileSource/>
                </m:MapTileLayer.TileSources>
        </m:MapTileLayer>
    </m:Map.Children>
</m:Map>

In the code-behind .cs file:

public class CustomTileSource : TileSource
{
        public CustomTileSource()
                : base(GetAbsoluteUrl("/ClientBin/Resources/{0}.png"))
        {
        }

        public override Uri GetUri(int x, int y, int zoomLevel)
        {
                var quadKey = new QuadKey(x, y, zoomLevel);
                return new Uri(String.Format(this.UriFormat, quadKey.Key));
        }

        public static string GetAbsoluteUrl(string strRelativePath)
        {
                if (string.IsNullOrEmpty(strRelativePath))
                        return strRelativePath;

                string strFullUrl;
                if (strRelativePath.StartsWith("http:", StringComparison.OrdinalIgnoreCase)
                  || strRelativePath.StartsWith("https:", StringComparison.OrdinalIgnoreCase)
                  || strRelativePath.StartsWith("file:", StringComparison.OrdinalIgnoreCase)
                  )
                {
                        //already absolute
                        strFullUrl = strRelativePath;
                }
                else
                {
                        //relative, need to convert to absolute
                        strFullUrl = System.Windows.Application.Current.Host.Source.AbsoluteUri;
                        if (strFullUrl.IndexOf("/ClientBin") > 0)
                                strFullUrl = strFullUrl.Substring(0, strFullUrl.IndexOf("/ClientBin")) + strRelativePath;
                }
                return strFullUrl;
        }
}

The code is from here

Bing Maps APIs Type

Microsoft provides multiple Bing Maps APIs:

1: Bing Maps Ajax Control   
The Bing Maps AJAX Control is a java script API that allows you to easily integrate maps and location functionality into your Web site. An Interactive SDK is also available at http://www.microsoft.com/maps/isdk/ajax, which offers hands-on, task-based demonstrations of features available in the version 6.3 map control release.

2: Bing Maps Silverlight Control
The Bing Maps Silverlight Control combines the power of Silverlight and Bing Maps to create an immersive mapping experience. You can find the Interactive Silverlight SDK at http://www.microsoft.com/maps/isdk/silverlight.

3: Bing Maps SOAP Services
The Bing Maps SOAP Services is a set of programmable SOAP services that allow you to integrate maps and imagery, driving directions, distance calculations, and other location intelligence into your applications, business processes, and Web sites.

4: Bing Maps REST Services
The Bing Maps REST Services can be used to perform tasks such as creating a map with pushpins, geocoding an address, retrieving imagery metadata or calculating a route.

5: Bing Spatial Data Services
The Bing Spatial Data Services includes the Geocode Dataflow API, which specifies URLs that you can use to geocode and reverse-geocode lists of spatial data.

From MSDN

Helpful Chinese articles about Silverlight Map SDK

Helpful Chinese articles about Silverlight Map SDK:

1: 【Silverlight】Bing Maps系列文章汇总

2: 【Silverlight】Bing Maps学习系列(七):使用Bing Maps的图片系统(Tile System)

3: 【Silverlight】Bing Maps学习系列(八):使用Bing Maps Silverlight Control加载自己部署的Google Maps

4: 通过必应地图SilverLight控件显示中国地图

5: 在必应地图SilverLight控件中加载其他地图(Google卫星图等)

6: 【Silverlight】Bing Maps开发应用与技巧四:自定义MapMode实现地图访问限制

All Bing Maps APIs

The Bing Maps APIs include:

1: Bing Maps Ajax Control   

The Bing Maps AJAX Control is a java script API that allows you to easily integrate maps and location functionality into your Web site. An Interactive SDK is also available at http://www.microsoft.com/maps/isdk/ajax, which offers hands-on, task-based demonstrations of features available in the version 6.3 map control release.

2: Bing Maps Silverlight Control   

The Bing Maps Silverlight Control combines the power of Silverlight and Bing Maps to create an immersive mapping experience. You can find the Interactive Silverlight SDK at http://www.microsoft.com/maps/isdk/silverlight.

3: Bing Maps SOAP Services   

The Bing Maps SOAP Services is a set of programmable SOAP services that allow you to integrate maps and imagery, driving directions, distance calculations, and other location intelligence into your applications, business processes, and Web sites.

4: Bing Maps REST Services   

The Bing Maps REST Services can be used to perform tasks such as creating a map with pushpins, geocoding an address, retrieving imagery metadata or calculating a route.

5: Bing Spatial Data Services   

The Bing Spatial Data Services includes the Geocode Dataflow API, which specifies URLs that you can use to geocode and reverse-geocode lists of spatial data.

More information please check MSDN site

Map API or Controls Licenses

Not all Map service is free, most of them need licenses, but they provide free using under some rules.

Here is example of Microsoft Silverlight maps control for Developers:

Developers at all types of organizations can use the Bing Maps Platform for free, under the following conditions:

    * Use is on public-facing, non-password protected Web sites
    * Limit of 125,000 sessions or 500,000 transactions in a 12 month period

Developers using the Bing Maps Platform for free receive:

    * Full choice of Bing Maps APIs
    * Aerial, road, and Streetside™
    * Access to documentation and developer articles on MSDN
    * Access to the developer forum for support questions

Google, Microsoft, ArcGIS all need license fee if the software deploy and customer use. But free for development.

1: Bing Maps Licensing and Pricing Information:  http://www.microsoft.com/maps/product/licensing.aspx

2: ArcGIS:  http://www.esri.com/software/mapping_for_everyone/api/pricing.html