The GVSU Writing Center has a list of handouts that can assist with many common writing concerns.
The Purdue Owl is an excellent online writing handbook and rhetoric.
Bedford/St Martin's Research and Documentation Online is excellent guide for documenting sources in a variety of citation styles.
College Commas is an online, interactive tutorial for comma usage.
Wierd Al Yankovic's Word Crimes is a fun video that pitches the importance of good grammar and mechanics.