Mecca

Most visitors to Mecca follow the set itinerary of the Hajj. Major sites include:

Sacred Mosque
al-Masjid al-Haram. The holiest site in Mecca and Islam. This mosque is built around the Kaaba.
Kaaba
At the center of the sacred mosque is this building, said to have been built by Prophet Abraham himself and his son, Prophet Ishmael. Covered in black cloth, it is circled seven times by Muslim pilgrims.

In addition to Makkah, sites involving Hajj include

Mina
The site of the symbolic stoning of the Devil.
Muzdalifah
The desert where the pilgrims spend the night.
Hill of Arafat and Jabal Rahma
The site of Prophet Muhammad's last sermon.
Jabal Al Noor
lit. The Mountain of Light. While its not part of the Hajj ritual this is still an important site; since atop this mountain is the famous Her'aa grotto, where Muslims believe Muhammed first had the Quran revealed to him.
Jabal Al Thur
This is the cave in which Prophet Muhammed hid in as he made Hijra to Yathrib Medina while being pursued by Meccans who were going to kill him. According to legend, the cave entrance was blocked by a spider which had cast a web to cover it.
Masjid e Taneem
This is a mosque which acts as a Miqat boundary for Umrah for people who have already just been in Makkah.
Hudaibiyah
It is situated on the old road connecting old Jeddah to Makkah. It's currently known as al-Syumaisi. This is the place where the famous 'Hudaibiyah Treaty' took place between the Muslims from Madinah led by Prophet Muhammed and the Quraish from Makkah. There is new masjid built next to the ruins of the old masjid.
Jannat ul Mualla
This is the cemetary in which companions during the time of Prophet Muhammed are buried.
Ja'aronah