Many people describe an increase in awareness of certain things -- usually based around their own beliefs, so the lists can vary.
There's mention of the 'blue pearl' in some cultures: an orb of blue light seen or sensed during meditation, representative of the opening of the third eye chakra. But this is not a necessity, in my opinion.
The third eye is merely a concept of awareness. Whether that awareness is of spiritual things, psychic things, the energy within things, etc., it doesn't seem to matter -- again, based solely on the beliefs of the person.