Here's one feature of ASP.NET 2.0 you may not have heard of- I certainly hadn't until it hit the blogs again recently.  If you drop a file named app_offline.htm in your web application's root folder, then all requests to that application will only return that file.  

Very handy for scheduled downtime, bringing down your app for updates, etc.  Scott Guthrie mentions the trick here, as well as a minor issue to be aware of when designing your app_offline page.

One novel way to implement this is to integrate it into your deployment process.  For example, a simple batch file like this:

copy C:\MyCompany\app_offline_updating.htm C:\InetPub\wwwroot\Demo\ProjectName\app_offline.htm /y

xcopy C:\MyCompany\ProjectName*.* C:\InetPub\wwwroot\Clients\Demo\ProjectName\ /s /q /y

del C:\InetPub\wwwroot\Clients\Demo\ProjectName\app_offline.htm

The batch file can be run manually, or even better- as part of your continuous integration process.  With a little javascript or meta-tag to do automatic refreshes, this is a great way to avoid errors and communicate downtime to users.