Is it necessary for linux? and if so, what is a good (preferably) free software to use?

no answers here--id also like to ask this question

Not neccessary...
While there are Linux virii in existence, they are few and far between and don't do much.
There are linux antivirii clients for scannign Windows partitions though, for that you might check out clamav.

To my knowledge the very few lilnux virus in existence have never gotten out into the wild. Only created and tested in labs. The biggest thing is not to go running around your box as root. Set up a user account and use it. Only use root if you absolutely need to. That way on the small chance something does get on your box, it can't do anything.

I would recommend a firewall though, as there is no reason in the world not to be running one.

Also check the md5sums and/or pgp signatures of the programs you download of the net. Make sure they are good. There have been cases where people have hacked into well known linux software sites and replaced it with there "own version". If I am just given a check sum I use two independent sources of what the md5 should be. If I'm given a pgp signature I just get the key from a keyserver and check it as realistically it can't be facked.

The only reason to be running a virus scanner in Linux is to prevent being a carrier of Windoze viruses (the plural is NOT virii). Othewise, don't even bother. If you feel really paranoid,... back up your user data frequently onto another partition, not owned by your user account.

u can install Clamav / Klamav (gui) if u want ... but just in order to protect Windows user from themselves.
I'm using it actually with kmail just to protect my friends against potential viruses.

Thank you all for the replies. This brings up a different question.

I run wpa_supplicant to access my wireless network, I have to be in root to execute the wpa_supplicant command and this runs in the background, is this a security risk?

also is there spyware out there that infests linux?

The virus and spyware question has been posted and answered many time here