Allowances in the US Internal

What allowances do employers commonly offer in the United States?

Generally, allowances are not commonly offered in the U.S. There are, however, some roles or positions where allowances are commonly given, e.g. automobile allowance for a sales position. 

Please note that all allowances are taxable in the U.S.

What should my company offer as a work-from-home (WFH) allowance in the United States? 

The amount of a WFH allowance is at your discretion. 

What considerations are there around meal allowances and reimbursements? 

Meals can be expensed as part of business travel/meetings. When meals are not part of business travel/meetings, a taxable allowance can be used instead.