I've used this software professionally, and even had it used on me. One of the most difficult to casually detect (IMHO as an I.T. consultant) is eBlaster.
I did some onsite consulting for a company in Texas this year and it took me two days to notice that they had surveillance software installed on the machine I was given to work from during my stay. I probably wouldn't have noticed at all if I had not dumped the Windows API table of the machine I was on.
It's perfectly legal to purchase this software and use it on your own machine. It can be a great recovery tool if your laptop is ever lost or stolen.
It's perfectly legal to use this software to monitor a child's internet activity. (Although by the time they are 14 or so, a clever child can defeat any of this software via a live CD)
It's perfectly legal for employers to use this software to monitor the actions of employees, provided they notify the employee at the time of hire.
It's perfectly legal for prisons to monitor the computer usage of inmates using this software
But what this company did was illegal despite the fact that they owned the computer in question.
The following states have legislation in place which under most circumstances prohibits the use of this genre of software on another adult without their knowledge or consent:
Alaska, Arizona, Arkansas, California, Florida, Georgia, Indiana, Iowa, Louisiana, Nevada, New Hampshire, Rhode Island, Texas, Utah, Virginia and Washington.
There's a widespread internet notion that, "If the computer is co-owned by two individuals, (i.e. A husband and wife) then one of the two can install the software without the knowledge or consent of the other."
Be careful though, especially if you live in one of the states listed above. The courts are not all agreeing with that idea. People (Mostly men so far) have actually gone to prison for installing this software on machines used by their spouse.
Regardless of what state you live in, using this software to intercept the email of another adult, including your spouse, can be actionable both civilly and criminally under 18 U.S.C. Sec 2511.