We posted on craigslist.org for a jr. java engineer and gotten the predictable hundreds of responses. I have seen all sorts of mistakes. Normally I wouldn't repeat this sort of information in a public forum, but what the heck, it's a blog.
Follow the directions. If it asks for a cover letter, send one. I kinda like this one: "Hi, Here is my resume. Thanks", but most people would find it too brief. Really long ones show you can't prioritize your work. Generic ones that don't match the listing generally disqualify you. If you're going to try to make the point that the job sounds "interesting", you MUST explain why. Nobody's done it yet. After reading it 100 times, "please email or call me at anytime" seems redundant.
Don't hide behind words. I'm only going to read 20 or so words when I skim your resume. The fewer words you have, the more likely I am to see the important ones. Have someone proofread. I'm waiting for a well-written letter. I don't care if you worked at the cineplex or best buy. Don't attempt more formatting than you can handle. Understand the limitations of MS Words or email before going wild with formatting buttons. It looks unprofessional if certain words or phrases are different sizes. Be aware that not everyone will have the same fonts and OS as you. One person put their resume into pdf, but somehow managed to completely mangle it.
Don't cc lots of companies. Think about the name of your resume. "oldresume.doc" is curious. WORST TRANSLATION
"References available on demand."
"...Hey, good luck with that. Ya know, you'll probably find someone too, market being what it is. Sucks."