All registrars have the "backorder" thing, so they will register the domain for you if the owner doesn't renew.
I was wondering how many of the total registered domains get renewed? Are there some statistics about this?
If it's a popular domain or rakes in money, very unlikely. http://www.norid.no/domeneregistrering/fornye-slette.en.html (That procedure is usually standard with many registrars, they usually send emails).
If it's a domain used for temporary services, like a "Free proxy" then of course it will be available after it expires as people are always looking for new domains.
But you also have to think of this from a moral point of view: If you're a small business struggling to stay afloat, and you forget to renew your domain, is it fair if someone buys it and runs their own site there? You should do a WHOIS on it then try to contact the owner yourself if they are selling it or going to renew it, etc.