Doujutsu
Eagle Eye (Washi-gan):
Washi-gan changes the eyes of the user to resemble those of an eacgle. Usually with no outward sign (in some it manifests as golden eyes while activated) it improves eye-sight to an unimaginable to most degree. It allows for telescopic vision with focus, but when in such a mode, if something interferes in a much closer range than the current 'calibration', it can cause trouble since a fly would be enlarged to monstrous proportions. Calibrating for short or long range sight takes a couple of seconds, which can prove crucial in combat. Another weakness is that while focused on a long-range target, anyone in a short distance would be effectively invisible, much like using binoculars. Regardless of range though, the user has perfect eye-sight. Due to its weaknesses it is mostly used from the air, where it is nearly impossible to sneak up to the user.
Heat sight:
Heat sight turns normal vision mode to seeing heat. Warmth is seen as 'warmer' colors while cold is seen as the 'colder' colors. It allows one to see through walls, if the source is strong enough and experienced users can even differenciate between the ground and air. Due to the difference from normal sight, it requires a lot of focus and training to be able to even walk normally in this way. One of its most serious weaknesses is sudden heat. A fire-based jutsu or an explosion that came too fast for the eyes to adjust, can and will blind the user for a short time. It is known to provide protection against some genjutsu since most users forget to calculate for heat. There is no outward change in appearence, though some shinobi might notice a more 'glazed' look in the eyes of the user. This ability is most useful defencively, especially in a perimeter since its next to impossible to escape notice from a user of this ability.
Echo-location:
Echo-location is not exactly a doujutsu, but it affects vision so it is placed there. In short it allows the user to emit and hear high-frequency sounds and by calculating the responce time of the sounds create and map out the surrounding area. A most usefull ability, especially to spies, but also a very rare one.