Answer:
There are literally dozens, if not hundreds. It would be an amazing feat to list them all.
Some of the more common ones:
AVG Free/AVG (Good one! But a bit slow)
Avira
BitDefender (good one!)
Panda
Trend Micro
Norton Antivirus*
McAfee
Avast
PC Pitstop (includes an antivirus)
These are all the ones I've seen on customers' computers so far.
* = I don't recommend it, personally. Many major, well-documented, well-known, well-hated issues for many years. My workplace refuses to install, sell, or re-install it on computers we're repairing. Installing it on a computer even on request of the customer *is a terminateable offense*. So needless to say, there's a lot of very strong reasons behind it.