The quick answer
System.Web.VirtualPathUtility.ToAbsolute("~/default.aspx");
Don't forget the ~/ (tilde) before the page name.
The lengthy explanation
Assume that we want to redirect to default.aspx that's on the site root:
Response.Redirect("default.aspx");
If we're on a page that's also on the root, it will map to http://www.mysite.com/default.aspx and work correctly.
The problem
The problem comes when we're not on the site root but inside a child folder. Let's say we're on a page that's inside the /Administration folder, the same line will map to http://www.mysite.com/Administration/default.aspx and that of course won't resolve and return a glorious 404: Page Not Found.
The solution
Solving this is easy using the ResolveUrl method:
Response.Redirect(ResolveUrl("~/default.aspx"));
Where this won't work
ResolveUrl is only available within the context of a Page or a UserControl; if you're, for instance, inside an ASHX, this method isn't available. In these cases, you have to use the following line to do the job:
System.Web.VirtualPathUtility.ToAbsolute("~/default.aspx");
We must include the tilde before the page name to make it a relative path, otherwise this method will throw an exception.
