Scratch

You could use these block images:
http://scratch.mit.edu/forums/help.php#bbcode - the images used to compose the forum blocks.

Another thing you might want to try is the following. In this example, I assume you are using Windows, with MS Office on it ('cause you said "powerpoints")

(1) Drag all of the scratch blocks into the script area of the same sprite. All at once.

(2) Right click on some empty space and select "clean up"

(3) Right click again but, this time, select "save picture of scripts". Enter "blocks" as the filename and browse for a folder to save in.

(4) Browse to the location where you saved the "blocks" image in the explorer. Now open the file "blocks.gif" with mspaint.

|  (5)Now select a block with the rectangular selection tool - make sure
|  you still have some gray around it. Press Ctrl-C to copy it.
| 
|  (6) Open you powerpoint presentation, go to a slide and press Ctrl-V to paste the 
|  block.
| 
|  (7) In the "picture toolbar", select the "set transparent color" tool and click on the
|  gray. The block is cropped out very nice.
|  Should work for office 2003 and older, not sure about office 2007 though...
|
|___ Repeat this part for each block

I know seven steps might look a lot, but I tried it out and it took me half a minute to see the first correct scratch block in powerpoint.

With this method, you can even snap them together *in* powerpoint... 

Does that helped?

Joren