There's heaps of safety apps out there. Check out Onside (www.onside.co.nz). Although it is aimed at the rural market it is a useful safety app in a range of workplaces. Has a risk register and an accident/incident reporting function. Can geofence a property and sign visitors/ contractors in and out.

really depends on what you are trying to achieve with the app? There are a number of available apps developed with the SME market in mind, like the one Graham mentioned or SaferMe (www.safer.me) which I understand is aimed towards SME contractors, or there are a number of companies using freely available messaging apps like Whatsapp to support their existing risk communication and reporting processes.
My suggestion would be to have a good discussion with your teams on what they need to support them, and then you'll have a good basis to assess any app against to evaluate if it will actually benefit your company/employees.

We have been using Safe365 for about 6 months now and find it ideal for workers out in the field. Incidents, risks and hazards can be loaded through a mobile device and submitted to the central website.
It's good for reporting and also has built in processes for improving performance.

Safe365 is relatively inexpensive and has the benefit of having the capability and culture assessment system built in. I've been using it for a couple of years and the addition of the app just makes the system even better.