If you don't send a cover letter or at least a little note , I assume you don't really care about the job. I actually figure you may not even remember you applied here. You spammed me.
I'm significantly more attracted to you if your cover letter demonstrates some knowledge about the company. Even better if you correctly show how your skills fit in. For god's sake at least say you looked at our website.
Give a little of your personality - some humor, a hobby, anything folks!
If your resume is not perfect for the job or if you don't meet all the requirements, tell me why I should consider you.
Use my name - look it up on the website, call the company and ask who will review resumes (spell the name right) .... whatever. Show me you are resourceful.
Follow up. Voice you enthusiasm in a creative, non nagging way, and make me look at your resume again. (For example, "Hi Matt, I recently saw an article about your company that made me even more excited and thought I'd check in"). It is often that I am overwhelmed. I may have seen and liked your resume but then got swamped.
Make it easy for me to hire you, not hard.