Kapersky: Strongest free/trial antivirus with best catch rate, larger system footprint. Often catches stuff you know for a fact is not a virus (incl. some legitimate programs), hence the "best" catch rate. Overdoes it.

Avast: Currently the best of the free antivirus programs, moderate footprint, good protection, ad supported, loud and noisy (but all of the sounds can be disabled)

Panda: I have had pretty bad experiences with this antivirus, but it is good for really out of date systems that can't handle the newer ones.

AVG Free: A little heavy at times, the more useful scheduling only in pro edition, version 8 took a light, free quality antivirus and made it oppressive and bloated. Not recommeded anymore.

(ESET) NOD32: Lightest system footprint for strong antivirus protection it provides, good catch rate, excellent database of viruses, expensive license (60 USD).

F-Secure: Good antivirus paired with a powerful firewall. Has found a lot of things the trial/free antivirus programs couldn't. Larger system footprint. Still couldn't find the nasty self-replicators though when I used it.

Symantec: Bloated, mediocre catch rate, severely overpriced, mediocre firewall (ZoneAlarm is free and does better), huge system hog. Drain on your computer, waste in your wallet.

McAfee: Mediocre, smaller system drain than Symantec, but worse catch rate.