How much damage does a lightning bolt deal?

My answer is slightly different. I put the /summon LightningBolt in a command block with a pressure plate on top and stood on it in survival mode. ...As seen in the photo five hearts of damage was taken, I didn't catch fire probably because it was raining. I also tested to see if the data was effected by the weather by testing this in a sunny condition. ...As in the first answer only (2 1/2) damage was taken. The reason for this is in sunny conditions only one lightning bolt was summoned. If hit in rainy weather the damage dealt will be more significant because more lightning can be summoned at one time.

Lightning striking on a player/mob will cause 5 damage (2 and a half hearts) of damage to them. Lightning will also set fire on the ground, which can cause the mob/player to light on fire.

How much damage does a lightning bolt deal?
5 damage or 2.5 hearts, and possibly igniting you or the ground you are on. The wiki is not so clear on the damage a player takes, but the other answers have proven this. You may also be struck multiple times.

Most entities struck by lightning are dealt 5 half hearts damage, and are set on fire which may cause additional damage. - Mincraft Wiki

When lightning strikes, all entities within a 6×6×12 region horizontally centered on the northwest corner of the target block with the bottom edge 3 below the target block are struck by lightning. Multiple passes are made over this region, so items dropped during an earlier pass may be destroyed during a subsequent pass; damage immunity usually prevents struck mobs from taking more than 5 half hearts damage.

Can it kill me with a direct hit?
Yes

If the player is killed by a lightning strike, the death message will read [player] was struck by lightning. - Mincraft Wiki