Jquery-texthighlight is a project mainly written in JavaScript, it's free.
Highlights text on the page if the GET parameter highlight is specified
This jQuery plugin can highlight text in a document.
add the script to any page and provide terms you want to highlight as values for the GET Parameter highlight
If you want to only hightlight text in a container, just override the default $.textHighlight.container = 'body'.
Example:
http://thomaspeklak.github.com/jquery-texthighlight?highlight=jquery;highlightTo toggle the highlight on/off you can use $.texthighlight.toggle()