Swords to Plowshares
Seriously, there is no reason for a deck with white to not run this. Exile for ONE! The life gain in negligible, especially when you get an Ulamog, the Infinite Gyre out of the way.
Path to Exile
See the above notes, except with "land fetch" replacing "life gain".
Wrath of God
BOOM! That is all.
Okay, now that the painfully obvious cards are out of the way...
This is instant-speed land fetch for 1. It may only be able to look for plains, but remember that the shock-lands from Ravnica have two basic land types as well as the original dual lands from Alpha. At the very least, this can look for up to 2 plains and put them into your hand at instant-speed.
Locking your opponents out of your turn? Ensuring that an opponent in a one-on-one has to do everything on their turn? Why would you not run this card? It's a bear that tells everyone to shut up!
It's a triggered Weathered Wayfarer, except free and better (despite the fact it only looks for basic lands). 'Nuff said.
Mother of Runes
This card makes your general fairly unblockable (unless your opponent has 2 colours of creature) but more importantly it protects your creatures. Instant-speed, free protection? Why not?
And now, a personal favourite:
Kind of like Grand Abolisher above, this card makes your opponents sad. Also, it makes people who run cards like Praetor's Grasp sad since it specifies "if an opponent would search a library...". It really slows down decks that require specific combo pieces as well as hoses decks that have a lot of search in them and that's why I like this card.
There are some cards that I consider essential in white EDHs.