The idea about writing some speculative letters is a good one, writing to possible employers which may have jobs that you don't know about has got to be worthwhile. After all, they may have already advertised and you didn't see their advert, or they may be about to advertise, in which case you might just get in first.
Typically a speculative letter might go like this:
I am writing to enquire whether your organization has any vacancies for which I may be considered. I am a reliable and responsible individual with 2 years of general office duties experience.
This experience includes answering the telephone, dealing with general enquiries, transferring calls, meeting and greeting visitors to the office, keeping the reception area clean and tidy, dealing with inbound/outbound post.
I am always eager to learn new skills and I am someone who works well in a team.
If you are looking for a new member of your office team, I hope you will contact me. I can attend an interview at your convenience.
So there is nothing wrong with writing a speculative letter like this, but a WORD OF CAUTION...
This is NOT the best approach for everybody.
When you write a speculative letter like this, you will probably want to send it to as many potential employers as you can on the basis that if you cast your net wide enough, you may catch something. That may be true and if all you want is a job (or ANY job) you could find it works for you, although it can be expensive mailing all those letters.
Ideally you would address, print and sign each one individually which can be quite tedious if you are sending out dozens of speculative letters, so don't cut corners by producing a standard 'Dear Sir' letter to photocopy as that will lose you a lot of credibility all at once.
I recommend that you only use this speculative letter approach for entry-level jobs or junior office type jobs which may be common to a number of organizations. A far better way for experienced or senior or older people is to understand and use the direct approach targeted letter - a radically different approach from what many people think of and is now the only way to succeed in today's tough job market. I'm sure you already know that there is so much more to getting a new job than just sending out hundreds of letters. However, you can make things happen by applying my direct approach which involves doing some careful research and targeting appropriate organizations by putting together a carefully crafted direct approach sales letter.
This type of letter is ten times more likely to get a response than any speculative letter that you might be tempted to send out.
Let me show you how to write an effective direct approach letter and give you a whole set of cover letter examples http://www.your-career-change.com/Cover-letter-examples.html that you can use freely for your own job applications.