If you're referring to PCs/laptops/netbooks, etc (running Windows) then the advice given on here by our recognised 'techie experts' used to be that Microsoft's own free security software was perfectly adequate. However Microsoft have now admitted (after it failed several independent tests) that Microsoft Security Essentials isn't as good as they'd like it to be. Windows 8 has it's own security built into it though and most users seem happy with it.
For many years I used AVG Free but I started to find that it was slowing my ancient computer down. So I switched to Microsoft Security Essentials but (along with the concerns I referred to above) I found that it's constant updates made everything grind to a halt on both this old XP machine (with very little memory) and on a far-better specified Windows 7 netbook. So I've switched again to the free version of Avast, which I'm happy with.