Some of the following may be specific to Quebecois French and as such should probably be avoided in conversation with people from France; after all, you don't want to be sneered at, do you? No indeed. Where there are no equivalent English idioms, and even where there are, some literal translations from the French have been included for posterity. Please feel free to /msg me with corrections, suggestions, and additions.

avoir d'autres chats à fouetter

"to have other cats to whip"; to have other fish to fry

avoir du chien

to have style or panache

avoir du toupet

to have nerve

avoir le coeur sur le main

to be generous

avoir le trac

to have stage fright

avoir les yeux plus grands que la ventre

to bite off more than one can chew

en avoir plein le dos

to be fed up with something

avoir quelqu'un dans le nez

"to have someone up one's nose"; to get one someone's nerves

en avoir ras le bol

to be fed up with

l'avoir sur le bout de la langue

to be on the tip of one's tongue

avoir un chat dans la gorge

"to have a cat in one's throat"; to have a frog in one's throat

avoir une faim de loup

"to have the hunger of a wolf"; to be famished

être à cheval sur les principes

to be a stickler for principles

être belle à croquer

"to be attractive enough to crunch"; to be as pretty as a picture

être au courant (de quelque chose)

to be informed (about something)

être dans de beaux draps

to be in a fine mess

ne pas être dans son assiette

"to not be in one's plate"; to be under the weather or out of sorts

brûler la chandelle par les deux bouts

to burn the candle at both ends

se casser, se creuser la tête

to rack one's brains

chercher la petite bête

"to search for the small beast"; to nitpick

couper les cheveux en quartre

to split hairs

coûter les yeux de la tête

"to cost the eyes from the head"; to cost an arm and a leg

faire du lêche-vitrine

"to lick windows"; to window-shop

faire la grasse matinée

to sleep in, to sleep late

faire la sourdre oreille

to turn a deaf ear

faire l'école buissonnière, sécher les cours

to skip class

faire marcher quelqu'un

to pull someone's leg

ne pas fermer l'oeil

not to sleep a wink

se lever du pied gauche

"to get up on the left foot"; to get up on the wrong side of the bed

mettre les pieds dans le plat

to put one's foot in it

manger sur le pouce

"to eat on the thumb"; to grab a bite

mettre sense dessus desous

to turn upside down

se mettre son trente et un

to dress to the nines

mettre sur pied

"to put on the foot"; to set up

passer une nuit blanche

to spend a sleepless night

passer un savon à quelqu'un

to rake someone over the coals, to tell someone off

prendre au pied de la lettre

to take literally

rester bouche bée

to have one's jaw drop (in surprise)

ne pas en revenir

to be unable to get over something (through surprise)

voir la vie en rose

to see the world through rose-coloured glasses

à la une

on the front page (of a newspaper)

aveugle somme une taupe

as blind as a bat

branché, dans le vent, cablé

trendy, with it

entre chien et loup

"between dog and wolf"; twilight

jusqu'au bout des ongles

"to the fingertips"; to the core

tiré par les cheveux

far-fetched

une drôle de tête

a funny face

de bouche à oreille

by word of mouth

À tes souhaits!

Gesundheit! God bless you!

cela m'est égal

it makes no difference, it's all the same

cela saut aux yeux

it's obvious

cela va de soi

that goes without saying; it stands to reason

c'est ma bête noir

that's my pet peeve

c'est le bouquet!

that's the last straw!

ce n'est pas la mer à boire

"it isn't the sea to drink"; it's not as bad as all that

cela n'a rien à voir avec...

it has nothing to do with...

il fait un temps de chien

it's foul weather

il pleut des cordes

it's raining cats and dogs

oui, quand les poules auront des dents

"yes, when chickens have teeth"; when pigs fly

plus on est de fous, plus on rit

the more, the merrier

revenons à nos moutons.

"let's get back to our sheep"; let's return to the subject at hand

si le coeur vous en dit

if you like

With many thanks to MALTP for formatting assistance.