The Canada Revenue Agency allows any reasonable business expense; that is, the item has to be appropriate to your business and used in an attempt to make money.
The CRA says business expenses are "certain costs that are reasonable for a particular type of business, and that are incurred for the purposes of earning income. Business expenses can be deducted for tax purposes. Personal, living, or other expenses not related to the business cannot be deducted for tax purposes."
There are some caveats to bear in mind when you’re thinking about claiming business expenses, however.
1) Note the word reasonable in the definition. What is a reasonable business expense for one business may not be for another. It may make perfect sense for a writer to claim ISP fees as a business expense, for example, but it seems odd for a bakery.
2) It's only the portion of a business expense that’s related directly to your business that can be deducted. If you buy and/or use vehicles or equipment for both personal and business purposes, you have to distinguish between the two and only claim the business portion.
3) All business expenses need to be supported by documentation. You have to have receipts to back up your business expense claims. For more on this, see Can I claim business expenses that I no longer have receipts for?
The Business Expenses Index lists many common business expenses and explains the income tax deductions rules relating to each expense.
If you're still worried or doubtful about claiming a particular business expense, talk to your accountant and/or give the Canada Revenue Agency a call.