The types of expenditure you can claim for through your limited company is largely dependent on the type of sector you are working in, builders for example are likely to have far more varied expenditure than say a management consultant who mainly works behind a desk all day.
HMRC's guide on claiming expenditure through your limited company says that for the expense to be claimable it must have been incurred wholly, exclusively and necessarily for the purpose of the business. In other words no trying to claim for a holiday to the Caribbean where you happened to take your laptop and do a little bit of work while you where there!
Genuine expenditure that the company incurs will reduce the profits of the company and in affect reduce the amount of Corporation Tax that will be payable to HMRC. Some expenditure however, such as entertainment of a client, will not assist with saving the company tax as per HMRC's rules.
Typical expenditure claimable by an average IT Consultant working through a limited company would be things such as....
Travel to and from your temporary place of work (if you are there for less than 24 months)
Travel to meet clients for business meetings
Mileage claims of 45 pence per mile for the first 10,000 miles of the year (25 pence thereafter)
Company insurances such as professional indemnity insurance
Hotel stays and subsistence for overnight business travel
Telephone and broadband costs
Stationary and postage
Computer equipment such as laptops, printers, software licences
Professional subscriptions such as IPSE membership
An allowance for using your home as your office
Accountancy fees and legal costs
Training courses and seminars
This is not an exhaustive list by far but is a general guide most contractors can use. If you are unsure if you can claim specific items through your limited company it is always best to run these past you accountant who will be able to give advice.