I don't normally correct people's spelling mistakes. I do so under two circumstances. One is when the writer puts an "(sp?)" after a word. They are signifying that they aren't sure of the spelling, in which case they wouldn't be offended by a correction. The other is when someone has written something I consider silly or simpleminded. In pointing that out to them, I will throw in a little something about their spelling, generally by using the word they misspelled and emphasizing it.
Why does it bother me so much? I would estimate 80-90% of what I read on the net is written by people I have never met offline. I know nothing about them apart from what they write. If what they write is illegible, what am I supposed to think? This is second-grade stuff. If you can't handle the stuff you were taught in second grade, how can I assume you learned anything that came afterwards?
With that in mind, here are the spelling problems that I see most often:
There is no such word as "alot". Don't ever write it. If you mean "divide among a group", the word is ALLOT, with two Ls. If you mean "very much" or "a large amount", it's A LOT, two words.
Look at that word again. D-E-F-I-N-I-T-E-L-Y. Do you see an A anyhwere? No. If you put an A anywhere in that word, you've spelled it wrong.
Again, there is no A in this word. Actually, this one doesn't bother me so much because there is a similar word, "resistance", that does have an A, so I can see where people can get confused. Isn't English wonderful? ;)
For some reason, whenever some people see a word that ends with an S, they feel compelled to stick an apostrophe in. Apostrophes are used to show possession or contraction. Period.
You know what homonyms are - words that sound the same, but are spelled differently and mean different things. "You're/your" is the biggest troublemaker, followed by "it's/its", "to/too/two" and "their/there/they're". Make sure you know the difference between these words, and make sure you use the right one.
Following these simple hints will make your net experience a more pleasant one for you and your readers. Thank you, and happy surfing! ;)