Steve Bardsley provides this checklist which newer (and some who are not-so-new!) divers might find useful. In Steve's own words, it's not absolutely comprehensive, but it's a pretty good start!

• Drysuit
• Undersuit
• Regulator/Gauges
• Stab Jacket
• Bootees
• Fins
• Hood
• Gloves
• Cylinder
• Weight belt
• Mask x 2
• Snorkel
• Torch
• Watch
• Dive Computer
• Whistle
• Knife/line cutter

Optional Extras:

• VHF radio
• Lifting bag
• Ankle-weights
• Delayed SMB & line
• SMB, reel and flag
• Camera
• Flares (personal & boat)
• Buddy line
• Strobe light
• Dive tables

Miscellaneous:

• Liquid soap
• Towel
• Money
• Food/Drink
• Small tool kit
• Talcum powder
• First aid kit

Clothing:

• Thick socks
• Boots
• Hat
• T-shirt
• Water/Windproof coat

Spares:

• Mask strap
• Fin straps
• O rings (assortment)
• Weights
• HP hose

Always tell someone where you're going and when you expect to be back