Automattic has released fine plugin for WordPress — WordPress Widgets which can be used to personalize your blog without knowing HTML. You don’t need to edit your WordPress theme at all! Just choose required widget, drag and drop it to sidebar. There are many widgets already created by different developers which you can use on your blog.
I failed to find widget which can get several chunks of HTML code and show one of them on sidebar therefor I decided to create it myself. You can download Ad Rotator plugin, then you need to unpack, copy adrotator.php to your BLOG_DIR/wp-content/plugins/widgets/ directory and activate it on Plugins page.
You can see the sample on this blog: there are several of my favourite books from Amazon on sidebar, and only one of them is shown at time.
Edit window looks like following:
You can enter title of the your widget, and HTML-code to display. Different chunks are separated with <--more--> code.
You could always download latest version of the plugin here.