I am also irritated by the improper usage of "your" and "you're" and of course "there", "their", and "they're". "They're" and "you're" aren't even pronounced the same as "there/their" and "your".

That leads to another thing that bothers me: when people mispronounce words. That is a pet peeve of mine, and I am equally annoyed when I do not know the pronunciation of a word. The words which tend to be mispronounced frequently include:

aye (pronounced like a long i; not "ae"),

yea (pronounced "yay"; not like the word "yeah"),

you're and they're (pronounced "you-er" and "they-er" respectively; not "yore" and "thare"),

voila (pronounced "vwah-luh"; not "wah-luh"),

our (this one I'm not sure about, but as far as I know it should be pronounced like "hour" rather than like "are"),

Thames (pronounced "temz"; not "thamez" or "tamez"),

and bury (I honestly don't know what is correct. I prefer "bar-ree" to "buh-ree, but "buh-ree" seems more likely to be correct).

Another pet peeve I have is the usage of phrases to describe something rather than using the designated term, such as using "the and symbol" instead of the word "ampersand" or "upside down E" as opposed to "schwa" (or "squiggly thing" instead of "tilde", etc.).