Where are the 13 Standing Stones?

Skyrim's got 13 standing stones erected in various spots around the province. They can be activated for various bonuses, and discovering them all nets an achievement (Standing Stones).

Where are each of these 13 stones located?

From the Unofficial Elder Scrolls Pages' wiki article on standing stones in Skyrim:

The Apprentice Stone

• On an island in a river just to the west of the line joining Morthal and Solitude.
• Those under the sign of The Apprentice recover Magicka faster, but are more susceptible to Magicka damage.

The Atronach Stone

• South of Windhelm, in the southernmost part of the hotwater zone, between an axe shaped lake and a 'u' shaped lake (closer to this one).
• Those under the sign of The Atronach absorb a portion of incoming spell damage and have a larger pool of Magicka, but recover it more slowly.

The Lady Stone

• In a lake, on an island directly north of Falkreath
• Those under the sign of The Lady regenerate Health and Stamina more quickly.

The Lord Stone

• East of Morthal, up on the mountain.
• Those under the sign of The Lord are more resistant to both Magicka and physical damage.

The Lover Stone

• East of Markarth
• Those under the sign of The Lover always feel a Lover's Comfort. (All skills improve 15% faster.)

The Mage Stone

• Southwest of Riverwood
• Those under the sign of the Mage will learn all magic skills 20% faster.

The Ritual Stone

• East of Whiterun, North of Whiterun Stormcloak Camp
• Once a day, those under the sign of The Ritual can reanimate nearby corpses to fight for them.

The Serpent Stone

• Directly east of College of Winterhold
• Once a day, those under the sign of The Serpent can use a ranged paralyzing poison on opponents.

The Shadow Stone

• South of Riften
• Once a day, those under the sign of The Shadow can become invisible for an extended period.

The Steed Stone

• West of Solitude.
• Those under the sign of The Steed can carry more and do not suffer a movement penalty from armor.

The Thief Stone

• Southwest of Riverwood
• Those under the sign of the Thief will learn all stealth skills 20% faster.

The Tower Stone

• Directly west of College of Winterhold
• Those under the sign of The Tower have the option to automatically open Expert or lower locks.

The Warrior Stone

• Southwest of Riverwood
• Those under the sign of the Warrior will learn all combat skills 20% faster.