Surface
Marker Buoy (SMB)
Used so that people on the surface can keep track of you.
Goody Bag
Where else are you going to put that lovely lobster you want for
dinner.
Torch
Essential item on a lot of dives in UK waters, these come in a wide
variety of shapes and sizes, they can use normal cells or be rechargable.
Dive Computer
Very useful piece of equipement, the basic ones will keep a track
of your time,
depth, any decompression requirements and alert you if your ascent
rate is too high. The top end ones can also monitor air consumption,
against air left to give you time allowances (with a big safety
margin!)
Buddy Line
When it's cold and murky sometimes a line (maybe 1-3m long) helps
you keep in touch with your buddy.
Decompression Buoy
Also known as a Delayed SMB (Surface marker buoy) can be released
to let surface cover know where you are, and to allow an easier
ascent/decompression (if required).
Gloves
An essential in UK waters for most people, normally made out of
neoprene.
Underwater Slate
Sometimes sign language is not enough!
Surface
Marker Buoy (SMB) 
time,
depth, any decompression requirements and alert you if your ascent
rate is too high. The top end ones can also monitor air consumption,
against air left to give you time allowances (with a big safety
margin!)
Gloves