Content and format
Speculative applications are generally structured as follows, though you may wish to adapt yours to your own style.
State why you chose to contact the company. Feel free to provide your research results on the company - the employer will be flattered.
Highlight your skills, abilities and strengths. Draw on your CV to help you here. Tailor this paragraph to the needs of the company or position that interests you.
Refer to your CV and highlight one or two situations demonstrating your professional competence in areas relevant to the company. It's always helpful if you can provide some kind of measure (figure, percentage, etc) for the results you have achieved.
Close by thanking the employer for taking the time to read your letter and CV.