Overview of cellphone spy programs
If you've never heard of cellphone spy programs, it is basically software that allows a parent to monitor their kid's cellphone calls or text messages. The same programs are being marketed to husbands and wives to check if their spouse is being unfaithful and they have cheating husbands or wives.. These spy programs can be the answer to your questions, "Do I have a cheating husband?", "Who does he always talk to on his cellphone?", "Is he cheating on me?" etc.
Let's start with the most basic type of program, since there are several brands, we'll just call it "cellphone spy 1". Now Cellphone spy 1's capabilities are limited, and usually cost less than $100. Cellphone spy 1 is the usual software program that allows you to read text messages from your cheating spouse's phone by taking his sim card, and attaching it to the device which will send it to your PC. This will not only let you read text messages in his/her inbox, but also text messages that he/she has already deleted. Yes, you read that right, text messages that they've DELETED already. Since most spouses that cheat delete their messages, this tool will show you EVERYTHING.
"Cellphone spy 2", this is just like the first type of software, but the difference is the cost and functions. It is similar because it will let you read the text messages, but it adds the ability to also listen in on the calls.
"Cellphone spy 3", these things are what you want if you're looking for total control over your cheating husband's cellphone. This software type of spy is usually (depends on the software you pick) undetectable. A cheating spouse , who is smart enough to know about cellphone spies, will NOT use their cellphone for talking with the person they are cheating on you with and when they do talk to this person, it will be in code. They could change the name of the girl to "Jack" or "John" to disguise it from you even if you have a cellphone spy to read his/her text messages.
What cellphone spy 3 does different, or more than the other 2, is that this type of cellphone spy will not only allow you to read text message, listen to voice calls, but also control your cheating spouse's cellphone. You can silently "call" their cellphone using this cellphone spy program, and listen to what is happening around your cheating spouse (as long as the cellphone is with them). If they turn off the cellphone, it will NOT actually be turned off, and you will still be able to listen in on whatever activities might be going on in the immediate area of the cellphone.
You can also get your cheating spouse's message/call archive, turn off his phone, have his phone call a specific number, play a ringtone, etc. You can literally control your cheating spouse's phone directly from you own phone. These types of cellphone spies are usually subscriptions based and cost $100+ PER month. I'd say you never really need all the features that a cell phone spy 3 type offers. It's just too much features. I would get the cheapest cellphone spy (type 1) that can read texts, which in my opinion would be more than suitable in helping you know whether or not your spouse is cheating, or having an emotional affair.
On the other hand, if you're here and want to know how you can STOP your cheating spouse from spying on your cellphone, simply remove the battery from the phone. Just turning it off will not work if you are being spied on with a program such as "Cellphone spy 3". As for the other cellphone spy programs (1 and 2), the cheap ones aren't much of a threat, just keep your cellphone with you all the time and don't let anyone touch it (so they can get the simcard) and they can't spy on you. Unless you fall asleep and they get to tamper with your phone, your private details are safe. Setting a password on your phone will also help prevent people from installing software to monitor your cellphone activities.
Cheating spouses who use different phones or hides cellphones you are unaware existed in the first place are difficult to catch. If you think your cheating husband or wife has a secret cellphone they use to keep things from you, try a different method to catch your cheating spouse. See the link on the sidebar for a guide to catch a cheating spouse.
Please read the comment section below before you ask your own question, the comment section in this post is very very helpful, and someone even posted their own guide below.
UPDATE January 2011
Because of all the comments asking somewhat similar questions, I have made mobile / cell phone spy follow up post with an actual recommendation, not just a guide to help you search for a cellphone spy. I discuss a cellphone spy software that works with most of the popular smartphones (iPhone/blackberry/android/symbian/etc), has SMS, GPS, and call tracking, the ability to look at your spouse's phonebook, all done remotely.
In that article I recommend software that are a combination of cellphone spy 1 and 3. It's like cellphone spy 1 in the fact that it is cheap and allows you to read text messages (remotely) and it's like cellphone spy 3 because it has GPS Tracking, is undetectable and lets you view everything on the phone. I didn't bother looking for cellphone spy software that actually lets you "silent call" the phone to spy on the surrounding area, because I think that is way too overkill and do not intend on recommending something above $100. It's just too much, you don't need that IMO, if you do want one feel free to do your own research and share with us what you are using through the comment section below.
Please remember that monitoring a cellphone YOU DON'T OWN is illegal. So is attaching a GPS device on a car you don't own. Any kind of monitoring you do should be on a piece of equipment or property that YOU DO OWN. You are responsible for your own actions and the actions of the people you hire (private investigators).