Didn't bother to read the other replies, but if they were not covered:
Could care less vs Couldn't care less -
couldn't is the correct usage. If you say "I could care less" then that means that there is still some wiggle room for how much you care.
Y'all vs Ya'll - the term is Southern slang. It means "you all" and the correct spelling is
y'all. Many people spell it ya'll which stands for "ya all."
Affect vs Effect - affect is usually a verb while effect is a noun (but not always). If you think of the terms alphabetically (a before e), there has to be an action (affect) before there can be a result (effect). Pollution can
affect the environment. The music produced a calming
effect.
Past vs Passed - passed is the past tense or the past participle of pass. An example would be "the car passed me on the left." Past is a different beast altogether as it can be a noun, adjective, adverb or preposition. It usually deals with time. An example would be "don't worry about the past."
Allusion vs Illusion - allusion is a reference to something such as a literary work. An illusion is a visual trick.
Alter vs Altar - alter means to change something. Altar is a place of worship or sacrifice.
Stationary vs Stationery - stationary (with an a) means not moving. Stationery (with an e) is paper that you write on.
Cemetery vs Cemetary - cemetery (e x3) means a graveyard. Cemetary with an 'a' is a misspelling.
Sell vs Sale vs Sail vs Cell - I will use examples to explain these: Do you want to
sell your car? The store is having a grand opening
sale. I want to
sail a ship. That is a small prison
cell. Bacteria is an example of a single
cell organism.
I could go on all night as a grammar Nazi...