closed as not constructive by R.K., Chad Cooper, Get Spatial, Devdatta Tengshe, iant♦ Nov 13 '12 at 15:17
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Well you can always use QGIS with its Python API to create a standalone application.
BruTile is a potential C# option - open source and active. That gets you into the C# / .net world (desktop applications, silverlight, etc). There are libraries that use BruTile that provide higher level abstractions - they are listed on the website.
If that doesn't help you, a much more detailed question might produce a more useful answer.