IF clients have Office installed:
I would recommend http://www.webdavsystem.com/server - this allows you to create the โvirtual file systemโ ( WebDAV , which is based on HTTP) that Office can get, thus allowing the client to open / edit / save the document (DOCX etc.) via HTTP (S) ... it is pure .NET, integrates perfectly with ASP.NET and even has a JavaScript-accessible API ... esp. The upcoming V 3 comes with a wizard that allows you to easily add this functionality to your ASP.NET application ... (just a happy client, not an affiliated one).
If clients do not have Office installed, then it becomes difficult:
For full support, you will need some kind of Silverlight / Flash / ActiveX / JavaApplet, which runs in the client browser and supports the necessary editing functions ...
I am not aware of such quality-quality components other than Microsoft: see http://www.microsoft.com/en-us/office365/online-software.aspx
You can integrate them into your web application, although they will not offer the full range of Office features in the browser ...
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