Who isn't worried about job security these days?  Even if you have a job with no apparent cutbacks expected, who really knows?  If things suddenly go south at your job, wouldn't you like to have the feeling of relative irreplaceability? 

I say "relative",  because no one is completely irreplaceable.  But you can make yourself such an integral part of the daily operation, that your boss would question how they would make it without you.  That would put you in a far better place than the rest of the staff when the going gets rough.

Here's a overview of an article on making yourself irreplaceable at work by Catherine Conlan of Monster.com

1. Stand out.

When you're in any work environment, it's critical that people know who you are and what you're doing.

2. Talk to your boss.

Instead of working hard to avoid your boss, you should actively communicate with him or her on a regular basis to develop a relationship and clarify expectations.

3. Don't be a drag.

People will go out of their way to avoid unpleasant co-workers. Even if your skills are at the top of their game, it's possible that a bad attitude can prove the difference when a department needs to cut someone.

4. Find your place.

Understanding your role within a department and company will help you make yourself indispensable to both.

5. Discover ways to develop and grow.

The more you invest in yourself to grow in your role, the more the company will see you as an investment.