<p>The interop library is slow and needs MS Office installed. 
Many times you don't want to install MS Office on servers.</p>

<p>I'd like to use <a href="http://en.wikipedia.org/wiki/Apache_POI" rel="nofollow noreferrer">Apache POI</a>, but I'm on .NET.</p>

<p>I need only to extract the text portion of the files, not creating nor "storing information" in Office files.</p>

<p>I need to tell you that I've got a very large document library, and I can't convert it to newer XML files.</p>

<p>I don't want to write a parser for the binaries files. 
A library like Apache POI does this for us. Unfortunately, it is only for the Java platform. Maybe I should consider writing this application in Java.</p>

<p>I am still not finding an open source alternative to POI in .NET, I think I'll write my own application in Java.</p>

What do you need to do with those file?  If you just want to stream them to the user, then the basic file streams are fine.  If you want to create new files (perhaps based on a template) to send to the user that the user can open in Office, there are a variety or
work-arounds
.
If you're actually keeping data in Office documents for use by your web site, you're doing it wrong.  Office documents, even Excel spreadsheets and access databases, are not really an appropriate choice for use with an interactive web site.