Hitachi Travelstar 5K100
This is a family of 5400RPM 2.5" hard disks manufactured by Hitachi. They are available in various sizes, and in both PATA (ATA-6) and SATA interfaces. Only the ATA-6 disks were shipped with ThinkPads though.
Models shipped with this disk
On some ThinkPad models, you must buy the IBM-branded version of this drive, since the Hitachi-branded version has an incompatible and unupgradable firmware (see Problem with non-ThinkPad hard disks).