• ^B -- Bold (it's obvious enough what this does)
• ^V -- Reversed text (try it and see)
• ^_ -- Underline (may need to hold down both Ctrl and Shift)

If you wanted to write the sentence, "This sentence has a highlighted word in it", with the word "highlighted" in bold, you would input it as below:

This sentence has a ^Bhighlighted^B word in it

If you wanted to put a word in as bold, reversed and underlined (these effects show up differently on different terminal-client program combinations), you would do it as below:

^V^B^_word^V^B^_

There is only one other control character that I know of that does anything. It is ^G. A ^G causes a beep from the computers of those receiving it. For example, if you type:

Can you hear this beep? ^G

everyone will get the text and a beep (if their client and terminal allow it). You can also use ^G in messages to people and basically anywhere in IRC, however, it is not considered polite to use beeps publicly as it may annoy people in the channel who are /away and trying to work or sleep or something. Therefore, you would be well advised to only use beeps in private messages of one kind or another.

One problem regularly discovered is that people cannot do bold text as Ctrl-B is already in use as Backspace for line editing, but this is easily cured. Type these two commands, where ^[letter] doesn't mean Ctrl-[letter], but actually two separate characters:

/bind ^b self_insert
/bind ^o backward_character

After this, you will be able to use bold text as explained above and the backspace for line editing will have been remapped to Ctrl-O. You can put these lines in your .ircrc file if you want (I would recommend it) if you leve out the / at the start of each line.