Several services offer to monitor your credit - supplying you with an updated credit report every month and your credit score, too.
Are they worth the monthly cost ($10-20 is the range I've most frequently seen)>
No, in the vast majority of cases. (I +1 with littleadv but I wanted to add another idea)
If you happen to have a reason to suspect somebody you know will be attempting to steal your identity as a means of revenge, having a monitoring service might provide you peace of mind. (Or you know you have been exposed via other criminal activity)
You can also get free monitoring from Credit Karma (which is run by Transunion I think). It won't be all inclusive as it is only one agency, but I don't see the harm.
But the best solution for almost everybody is to freeze your credit. Freezing your credit will prevent the most harmful shenanigans that a credit monitoring service will only be able to tell you about after the fact. Freezing is typically cheaper as well. You can't have credit opened in your name you don't know about, so monitoring your existing statements and a freeze will keep you as protected as any service I know of.
Personally I use CreditKarma. It doesn't claim to give you your FICO score, but it is free, has credit monitoring, and displays two credit scores: a Vantage Score, and TransRisk score. I'm not sure if it is as good as the pay for companies since I've never used one, but for me it has worked well, within 24 hours of applying for a loan I received an email telling me there was a new inquiry on my account.
Combine that with the three credit reports littleadv linked to, and I am set.
But you're not buying the credit monitoring service, you're buying your peace of mind.
You can get a credit report every 4 months for free (annually from each of the credit reporting agencies, so spread it out over the year). But.... I, for example, got my wallet stolen a while ago, so I prefer paying $150 a year (AMEX) for the monitoring service for the next couple of years, for my peace of mind.