Does anyone know if there's a tool online that allows for selective highlight of multiple regions of text?

For example, if I'm reading an article while writing an essay and want to remember key words or sentences within the paper, is there an online tool with Internet Explorer 8 (or any web browser platform) where I can highlight within the text, simply to highlight, and in multiple places (Ex: when I highlight a sentence, and take my finger off the mouse, and then go back to highlight another sentence, it won't get rid of the previously highlighted selection) HELP!