A mobile phone - obvious really.

A Tent to sleep in - obvious really (* see below).

Walking Boots - the countryside can be rough and wet. Bring a compass and get a good map if you intend to go hiking.

Vehicles - it will be quite a challenge getting around from A to B, especially if public transport is full to capacity or closed by the police, so if you can bring a vehicle, do so.

Waterproofs - it will be sunny but it might rain in Scotland, in fact it rains a lot in scotland and the weather can change very quickly. It will still be sunny, so maybe bring suncream :-)

Insect Repellant - the 'midges' in the countryside can reduce a grown man to tears. One of the best defences believe it or not is Avon 'Skin So Soft Soft & Fresh Dry Oil Body Spray'.
NB police are unlikely to us CS gas so normal recommendations like in europe and the US etc not to wear oil based products like sun cream does not apply.

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This is a basic list for people wanting to come to scotland.

There are lots of lists of what you should bring for protests etc that include things like first aid kits, maps, bottles of water, food for the day etc

There is also legal issues about what to bring - for example unlike most of Europe there is no need to carry ID or Passports in Scotland / England. For people who are camping it's worth noting that while it is not unlawful to carry a folding penknife of up to three inches long (76 mm), it could still be construed as an offensive weapon depending on circumstances. Knives that flick in to position or that lock are not penknives. It is best not to carry any knife or bladed item at all.

* It's also worth noting that wild camping and squatting buildings is illegal in Scotland, however it is expected that there will be several campsites set up for accommodation.

For more information on these and other issues see:
 http://www.g8legalsupport.info/guide