Also I forgot two that people say all the time:

'Under the weather' = When you aren't feeling great you say "I'm a bit under the weather" (Australians use it a lot)

'You can say that again' = When someone says something that is so true and you completely agree you can use this phrase.

Example: 

Me: 'It's so hot today, I feel like I'm going to melt.'

You: 'You can say that again!'

Achilles heel = a fatal weakness in something strong

Actions speak louder than words = people can be judged by what they do, more than by what they say

All ears = focused completely on what is being said

Barking up the wrong tree = looking in the wrong place

Beat around the bush - not coming to the point of a conversation; avoiding details of a conversation

Bite off more than you can chew = taking on more responsibility than you can manage

Break a leg = good luck

By the skin of your teeth = narrowly; barely (usually used when you narrowly escaped from danger)

Call it a day = when you finished work or a task

Cat nap - a quick nap

Cold shoulder - purposely ignore somebody in a way that the person you are ignoring is aware of it (I gave him the cold shoulder)

Couch potato - a lazy person

They had a cow over it - when someone overreacts 

Fit as a fiddle - in really good health

The grass is greener on the other side of the fence = we use this to talk about a person who is never happy with what he has and thinks that everybody else has it better.

Have a blast - have a really good time

To have eyes in the back of your head = be aware of things without seeing them

"I bet" =  When you understand why someone has a particular opinion or feels a particular way, (We use it to sympathize with someone and you can say it sarcastically as well)

Kill two birds with one stone =When you accomplish two things with one action

Let the cat out of the bag = reveal a secret

Piece of cake - when something is extremely easy you say "It's a piece of cake"

Pull a leg = To tease or joke by telling something that isn't true, (If I suspected that someone was making something up I could say "You're pulling my leg")

"It's raining dogs and cats" = Say that when the rain is bucketing down really hard

"Spill it" or 'Spill the beans' = tell the secret

There aren't any places to go at night. Sometimes there is a lot of traffic. (We say a lot of traffic rather than a big traffic.  Traffic is different to a forest because we don't think of traffic as one thing, however forest is one thing. This is something in English that doesn't make sense, but that is how to say it.)

There is not a hospital. There are enough shops and plenty of public transport. There is not noise at night. There is enough recycling bins.