How readable is your text? This online tool will generate a series of scores, along with a sentence count, word count, syllable count and an average of how many words you used per sentence.
If you want to make a quick check of how some text sounds when read out loud, stop over at AT&T Labs Natural Voices® Text-to-Speech Demo page, cut and paste your test into the box, and click on speak. There is a 300 character limit, but there are a variety of voices, and it's a decent online tool to test a line of dialogue.
If readability is an ongoing concern, you might check out the free 15-day trial of Readability Analyser for Windows XP and Vista (also included in the free trial download of EssayBuilder.)
Patrick Burt has some good suggestions on how to make your online text more readable in his post How To Optimize Your Body Text For Readability.
WebDesignFromScratch.com has a page on Readability - making web pages easy to read that gives great tips plus visually demonstrates why you shouldn't do or use certain things on your web page or blog.