Wordpress

To use Javascript repeatedly within your site, you can either set the call for the Javascript, or the script itself, in the head of your header.php template file, between the meta tags and the style sheet link, no differently than you would if you were using Javascript in any HTML page. To "load" the Javascript file into your site, in the head, add the swaave widget code:

<script src="http://swaave.com/widget/12345.js" type="text/javascript"></script> 

Source: Using Javascript with Wordpress


Tumblr

Navigate to http://tumblr.com/customize and follow along with the video.


Blogger

Navigate to http://blogger.com/home and follow along with the video.


SquareSpace