Find out about what the employer does. If you are going to write to them then you do need to know something about the company
Do they have a website? Are there any press reports that you can refer to? Have they got a new product, new big contract or have they just taken over another company? If you can mention any of this in your covering letter it shows that you have read up on them; and you could use the information as a reason to suggest why they may need someone like you.