DISCLAIMER: Nothing available online is a legitimate substitute for actually coming to the Writing Center and getting some one-on-one help. The writing center tutors are knowledgeable and friendly, and could save most students a lot of time and anguish, answering unique questions or offering special suggestions that would be otherwise difficult -- if not impossible -- to find online.
That being said: If you need an answer to a writing question, the absolute best source on the web is the Purdue Online Writing Lab, or the OWL. I utilized this resource as an undergraduate -- for both my majors, economics and English -- and I have pointed many other writers and students to OWL as a writing tutor for the last five years. My professors suggest it, my colleagues swear by it, and Google results it (go ahead, google "owl" -- that's right, it's more popular than Wikipedia's owl entry).
Want to know how to write a good cover letter for your next job application?
OWL has it.
Want to know about commas and lists?
OWL will explain it, and then explain it more -- Oxford comma style.
Can't understand APA citation rules?
OWL knows that answer too, and will tell it for free.
Want to know the exact day and time in which you will Understand?
OWL knows this too, but has told only me.
In short, the OWL website is the best thing to come out of Purdue since Kyle Orton, zing, and is a viable resource for EVERY writer -- from an economics graduate student like myself to an avid blogger just trying to figure out some handy grammar rules. Use it!
Bradley Woodrum also writes for Homebody and Woman and Cubs Stats.