syntax Highlighting on wordpress.com blog

you can use the [sourcecode] shortcode on WordPress.com which is built in for you, no need to install or configure anything.

For example to get this results :

enter image description here

just wrap your code in these tags:

[sourcecode language="css"]
your code here
[/sourcecode]

you can see a list of supported languages and a few other parameters you can use with this shortcode at http://en.support.wordpress.com/code/posting-source-code/