I did a post last year called If You're Using XCopy, You're Doing It Wrong that also included a video of my talk at Mix10 where I show how to deploy website with Web Deploy. One of the cooler not-very-well-known features is called Web.config Transformation. Once folks see it, then immediately want a general solution.
First, from the previous post:
You can right-click on your web.config and click "Add Config Transforms." When you do this, you'll get a web.debug.config and a web.release.config. You can make a web.whatever.config if you like, as long as the name lines up with a configuration profile. These files are just the changes you want made, not a complete copy of your web.config.
You might think you'd want to use XSLT to transform a web.config, but while they feels intuitively right it's actually very verbose.
Here's two transforms, one using XSLT and the same one using the XML Document Transform syntax/namespace. As with all things there's multiple ways in XSLT to do this, but you get the general idea. XSLT is a generalized tree transformation language, while this deployment one is optimized for a specific subset of common scenarios. But, the cool part is that each XDT transform is a .NET plugin, so you can make your own.
<?xml version="1.0" ?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
<xsl:attribute name="value">New Setting Value</xsl:attribute>
Or the same thing via the deployment transform:
<add name="NewSetting" value="New Setting Value" xdt:Transform="Insert"/>
This kind of config file transformation is so useful in fact, that it's one of the #1 feature requests...as a generalized solution. Folks want to transform their app.configs, or any XML file as part of their builds. Additionally, the current system only runs the transforms as a part of the publish process and folks would rather the transform happen "on F5" or on build. So, my team members Sayed Ibrahim Hashimi and Chuck England have done just that as a small VSiX called SlowCheetah XML Transforms.
In this screenshot I've created a simple Console Application, made a basic app.config, then right clicked and selected "Add Transform." This gives an app.debug.config and