Here is one of my favorite people in the world talking about serial commas and the sometimes contentious "Oxford Comma."
There is a rather loud video ad on this page, so you could turn your volume down.
http://www.quickanddirtytips.com/education/grammar/serial-comma?page=all
Here, again, is Grammar Girl. And, again, threre is a rather loud video ad on this page, so you could turn your volume down.
http://www.quickanddirtytips.com/education/grammar/oxford-comma-pictures
Identifying Incorrect Sentences (fragments, run-ons, and comma splices)
http://www.wisc-online.com/Objects/ViewObject.aspx?ID=WCN7104
Run-on Exercises
http://depts.dyc.edu/learningcenter/owl/exercises/run-ons_ex1.htm
http://depts.dyc.edu/learningcenter/owl/exercises/run-ons_ex2.htm
Fragment Exercise
http://depts.dyc.edu/learningcenter/owl/exercises/fragments_ex1.htm
Here is a short article about the dreaded and horrible comma splice. You can listen to the article read by the author by clicking the audio near the top.
http://grammar.quickanddirtytips.com/comma-splice.aspx
Here are online exercises concerning the terrible comma splice:
http://www.bristol.ac.uk/arts/exercises/grammar/grammar_tutorial/page_47.htm
http://depts.dyc.edu/learningcenter/owl/exercises/comma_splices_ex1.htm
http://depts.dyc.edu/learningcenter/owl/exercises/comma_splices_ex2.htm
My apologies for misspelling "difference" in this movie. One day I will correct it and re-record the audio, but I hope you can forgive me if I haven't yet.
Watch out for the loud video ad again!
http://www.quickanddirtytips.com/education/grammar/why-commas-matter?page=all