Hero Sight Range
The Sight Range of most of the heroes is 1800(day) / 800(night). There are some exceptions:
Aiushtha, Enchantress: 1800 / 1200
Rooftrellen, Treant Protector: 1800 / 1200
Gondar, Bounty Hunter: 1800 / 1000
Traxex, Drow Ranger: 1800 / 1700
Mirana Nightshade, Priestess of the Moon: 1800 / 1200
Puck, Faerie Dragon: 1800 / 1200
Leshrac the Malicious, Tormented Soul: 1800 / 900
Balanar, Night Stalker: 1200 / 1800. So you now know that balanar can see you at night even when you can't.
Meepo, Geomancer: 1800 / 1800. Ganking a nice meepo is never easy.
Alleria, Windrunner: 1800 / 1200
Skills which grant enemy hero sight
These skills "mark" an enemy unit for a period of time.
Slithereen Guard's Amplify damage
Dwarven Sniper's Assassinate
Spiritbreaker's Charge of Darkness
Alchemist's Unstable Concoction
Bounty Hunter's Track
Lord of Olympia's Thundergod's Wrath
Bloodseeker's Strygwyr's Thirst
Skills which give extra sight at a certain place
Dwarven Sniper's Scattershot
Goblin Techies's Land Mine, Stasis Trap, Remote Mines
Templar Assassin's Psionic Trap (Use at rune spots and roshan)
Treant Protector's Eyes in the Forest
Broodmother's Spin Web
Goblin Clockwerk's Rocket Flare (Used to check rune spawning)
Spectre's Spectral Dagger (Used to predict ganks)
Priestess of the Moon's Elune's Arrow
Keeper of the Light's Illuminate
Items which give extra sight
Bloodstone if charged, gives sight of the place where the owner of the item died.
Sentry Ward: 200/ 200. (700/ 700 True Sight range)
Observer Ward: 1600/ 1600.
Animal Courier / Flying Courier.
Shiva's Guard: The projectiles have 800/600 vision.