Blizzhackers

I love this subject :p

Here's an open source C# project
but you can just do the win call in any program to make it "overlay"
as long as you know how to make the window semi transparent you're good to go
but there's a way to make a window not capture any mouse input and to have it fall through the window to the back window.

http://www.aobot.org/forum/viewtopic.ph ... ent#p96721
That post contains the links to the svn

I created a follow class (in utilities) which checks the locations of the overlay window with the game window (two given windows) and makes sure to either move the overlay to the game or visa versa (ie: move the game to a control window)

this is the part of the source though you'll need
http://code.google.com/p/aomap/source/b ... rol.cs#189

You can look into the MS api for that and look @ all the different extended values and what not so you get an idea of what those numbers are doing.

Good luck ; )
That's how I've been doing it
using a normal window and the C# GDI / other methods
Although I hear if you use dx3d you can get much better frame rates
although you still need the game in a non full screen mode for this to work
but it's not intrusive like injecting a dll
There's also a few map clients like this for SC2 that have came out

What were you going to use it for?