the 3 main ones (free) that most people use are
Adware
Spybot search & Destroy
and
malwarebytes.
There are others, such as Mcafee, and Norton. They tend to want to sell you a "package" - antivirus and spyware.
I have all 3 on my system, as each 1 tends to have strengths over the other in certain areas.
If you are willing to pony up to pay - that's where you get the real benefit, because they will be pro-active, instead of re-active (free versions). Spybot will do some pro-active searching, but will not catch everything.