You can use the current page node, either in a drawing or in positioning a drawing.
I will show how to position a drawing to the top left corner of a page, using a node which contains a TikZ drawing. The benefit is, that the inner picture doesn't need to know about the page position at all.
A specialty is, that nodes of the inner drawing inherit options, such as the anchor, from the parent node. So see, how I took care: I stated the node anchor style in the inner drawing, even if center is the default.
\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[remember picture, overlay, transform shape]
\node [anchor=north west, inner sep=0pt]
at (current page.north west)
{
\begin{tikzpicture}[every node/.append style = {anchor=center}]
\begin{scope}[blend group = soft light]
\fill[red!30!white] ( 90:1.2) circle (2);
\fill[green!30!white] (210:1.2) circle (2);
\fill[blue!30!white] (330:1.2) circle (2);
\end{scope}
\node at ( 90:2) {Typography};
\node at ( 210:2) {Design};
\node at ( 330:2) {Coding};
\node [font=\Large] {\LaTeX};
\end{tikzpicture}
};
\end{tikzpicture}
\end{document}

I took the example from my upcoming LaTeX book, as it was easy for me to grab from my hard disk.
current pagenode (p.250, pgfmanual). – Paul Gaborit Aug 07 '15 at 11:41