eDonkey and eMule
You can get the free version of eDonkey from here (scroll right down to find it)
You can get eMule free from here (select the mirror nearest to where you live)
I downloaded eMule and eDonkey and found eDonkey a lot better but many people swear by eMule so it's really up to you.
Obviously many people already have these programs thats why they are currently the best but if your behind everyone (like I was) get them from the links.
Unfortunaely the way p2p works by next year most people will probably be using completely different program from eDonkey or eMule so get it while it is still the best.
A good website for free downloads is download.com
And if you've ever wanted to become a hacker check out this site for instructions. It explains the lifestyle and culture of a hacker as well as different programs used