I don't have the market cornered on these kinds of words, but here are several more I've discovered in various works of fan fiction:

I won't remember all the words I want to highlight here, so I'll create a series of posts with them and slap on a label to connect them.

These are words I've seen confused in my perusal of amateur fiction-posting websites. Lack of spell-check doesn't account for many of these; laziness does. Some of these just blow my mind, but considering what my children have been taught about grammar (next to nothing) I shouldn't be surprised.

there- most commonly used as an adverb; in a particular place, point or manner
their- possessive pronoun; Their child.
they're- contraction of they are

bear- verb; hold up, bring forth, produce
bare- adjective; without covering, open to view, plain, scarcely

throne- noun; chair or seat occupied by an exalted person, the office or power of such
thrown- verb, past form of throw

reign- noun/verb; period of time a sovereign occupies a throne, a dominating power
rein- noun/verb; leather strap connected to a bridle to control a horse, to control

peek- verb/noun; to look quickly / a quick look
peak- noun; the pointed top of anything, highest point or level
pique- verb; to wound (pride), excite, provoke

to- preposition; used to express motion or direction toward a person, place or thing
too- adverb; also, in addition, more, very
two- noun; the number 2

affect- verb/noun; to act on, move the feelings of / feeling or emotion
effect- noun/verb; result, influence, meaning, purpose/ to make happen

your- possessive pronoun, second person (Your wallet.) 
you're- contraction of "you are" (You're coming tonight.)

led- verb, past tense of lead (pronounced "leed"- see below)
lead- verb; to go before, guide, influence, to take or bring (also a soft metal pronounced like "led")

I think that's enough for now. It is amazing how many frequently confused words aren't even spelled the same. Regional pronunciations are more to blame than anything, I think. 

I thought it might be helpful to put in a description of parts of speech, since they are a main part of grammar. I'm going to post a bit later about confusing words and knowing what I'm talking about would help. Also, I'd like a reference for myself. :) The following chart is from an ESL website called easypacelearning.com.

I suppose I should have introduced myself first. I'm AJ. I currently write fan fiction at fanfiction.net and LiveJournal. I am constantly looking for ways to improve my own writing and I aim to do that by helping everyone else along with me. At least other English-speakers.

At least at writetodone.com. Granted, the quiz is only 5 questions. :) Their quiz is here.

At any rate, I can't help but pick out grammatical errors wherever and whatever I read. Not that I'm perfect, but they just pop out at me. Especially commonly confused words, like affect and effect. Verb and noun, respectively, argh. I actually like to hang out at Dictionary.com, and I did used to read the dictionary as a kid.

So my main idea behind this is to put grammar tips here so I can remember them. And if I can help anybody else along the way, fantastic! Thank you and welcome! :)