Playing D&D4e, I have a situation in mind where a character could reasonably WANT to get hit with a normally harmful attack. Is there anything in the rules allowing the character to purposefully take the hit, not even bothering to attempt to defend or dodge out of the way of it?
For example, a dragonborn character makes a breath attack that includes an ally in its blast. The dragonborn has the Nusemnee's Atonement feat, which allows a player whose AOE damages an ally to take the ally's damage instead. This can be paired with a dragonborn feat that recharges your racial dragonbreath power when you take damage of the type it deals. This combination lets the dragonborn recharge their dragonbreath as long as they hit an ally with it. Normally you'd want your ally to at least have a chance to dodge your AOE first, but in this combo, it's best if they can allow themselves to get hit!
Can the ally choose not to defend against the attack, purposefully taking the hit, knowing that the character making the attack will take the damage in his place?
Although the option to give up and be auto-hit is not explicitly denied, power descriptions are written assuming that threats are real and that all combatants are using their best efforts to hit and avoid being hit. For instance, the allocation of so many hit points of damage, as opposed to just dying when struck with lethal force, is because even when "hit" it is assumed you have deflected or avoided most of the blow, and are only truly vulnerable when all the energy to fight has gone from you.
Some powers work less well, or can be abused, if these assumptions are not played out.
The equivalent rule in the positive sense - that to gain the benefits associated with hitting an enemy, it has to be a real attack - is called the "Bag of Rats" rule.
There is no RAW for a character directly allowing themselves to be struck by an ally in order to gain a side benefit. In fact the assumption in the game rules is that even when they have been "hit" that they have managed to prevent themselves being outright killed by actively resisting or avoiding the attack.
With that interpretation, a particularly harsh ruling might say that character that simply gave up and allowed themselves to be struck by a lethal attack such as their ally's breath weapon should be killed outright (and using the Atonement should have a similar effect on the Dragonborn). The thing that prevents PCs being similar to minions in this respect is their pool of hit points that represents the character resisting this end result.
Just to be clear, I am not advocating that ruling, just pointing out that it would be logically consistent with game design.
I think that a character/team build that relies on this effect needs discussion with a DM before using it in play, as it is subject to DM interpretation of the rules. wax eagle suggested in comments that it might be reasonable to have the Dragonborn's ally expend an action to grant Combat Advantage to them, in order to set up the self-recharge.
Where you end up on scale from "fun tactic" to "over-optimised weirdness" I cannot judge - hopefully you have a DM who's willing to experiment and you can give it a try and figure out something that's fun for the group.