Recently did a little bit of playing with querystrings (at the end of the URL, that is after the .aspx, put a '?' then the value below, to use more than one use an '&'), thought it might be something other people are interested in. Strange that I haven't really stumbled across these documented anywhere else, it might just be that I haven't been reading the right documents! (I'm sure that if this is the case it will be politely pointed out to me!)

Let me know if you have any others and I will add them to the list.

Querystring Function
Contents=1

This is probably the most useful of querystrings. Using this on any "Web Part Page" will take you to what is called the "Web Part Page Maintenance" page (screenshot below) which allows you to delete web parts from both personal and shared views. This feature is especially useful if you have added a misbehaving web part. In this situation you can find yourself in a "chicken and egg" situation, whereby you can’t access the page to remove the web part because the web part you have added is causing an error on the page.

Note: I have actually come across a situation where even this technique couldn’t remove a misbehaving web part, in this situation FrontPage becomes the only option.

Mode=Edit This flips a "Web Part Page" into edit mode, its the same as clicking on "Edit Page"
Mode=View This flips a "Web Part Page" into view mode, it’s the same as clicking on "View Page"
PageView=Personal Displays your Personal view of a "Web Part Page", this is the same as selecting "Personal View" from the "Modify Shared/My Page" dropdown.
PageView=Shared Displays the Shared view of a web part page, this is the same as selecting “Personal View” from the "Modify Shared/My Page" dropdown.
ToolPaneView=2 Shows the "Add Web Parts" toolpane, allowing you to drag and drop web parts onto a "Web Part Page".
ToolPaneView=3 Shows the "Search" toolpane, letting you search through the web part galleries before adding a web part to the page.

Finally (there is always a finally isn't there?) a couple of things that should be considered:
1. These querystrings *will* respect the underlying SharePoint permissions, so for example, someone without the ability to add web parts will not suddenly be able to by using these querystrings.
2. These will let you add web part to pages that may not have been designed to have web parts added to them, for example the "Sites" page. Be very careful as there are certainly some issues to be discovered, don't try and get too fancy!
3. You use these at your own risk.