I might add that while one may use AVG's free version, I would urge people to buy the paid version- Not that it offers anything more than the free, it is a matter of supporting the good folks at Grisoft.
A good portion of the software recommended on this thread is free to use. The ethical statement offered by the various authors is to provide great software to everyone regardless of their ability to pay. Therefore, it is incumbent upon those of us who can pay to support these great works.
If a software found freely on the internet has been a good and faithful work that has reliably performed a needful service, please take the time to go to the author's page and find their "donate" button, or buy their paid version- Your contributions keep these programs free to those who need them, and fund the improvements yet to come.
Supporting those who make the software is one thing, paying a third party to give you the free version when no money goes to the creators is quite another. If it's AVG, purchase it directly from Grisoft -- not from a third-party.