In my Mandarin class there was a poster on the wall obviously aimed at people learning english, with the following colloquialisms..

can't make head or tail of what he's saying
got the wrong end of the stick
talking at cross-purposes
in a nutshell
speaking your mind
can't get a word in edgeways
let's wrap up the discussion

Ratewhore - someone who changes banks for cheaper fees. (source Campbell live)
Idolspise - a mixture of envy and idolatry (source Dompost)

I put together some not-so slangy idioms recently:

on the other hand
off track / on track / get back on track
out of your head
get on your nerves
under the table (deals)
don't hold your breath
hit the mark
go to town
strike while the iron's hot
be under the weather
get up your nose
get out of line / get back in line
never fear
neck and neck
off the record
for the record
set the record straight
on thin ice
sweep [something] under the carpet
run a tight ship
listen to (the voice of) reason
see the light
come to your senses
learn [something] by heart
put your heart into [something]
mind your own business
make your mind up
learn the ropes

I find the more I think about idioms the more confused I get. Where do you draw the line? I start buzzing out on linguistics terms like illocutionary and perlocutionary force. Hot diggety!