The following sites have some great clipart available

Clipart.com -

Doesn't have clipart in the site itself, but maintains one of the largest list of clipart related sites on the internet. A must for clipart lovers.

WebCavern.com

Is a site containing literally hundreds of links to webmaster resources. Tons of graphic sites, and sites that webmasters would find useful. A definite must.

Likewise http://www.Freegr.com    Is free, and has hundreds of graphics, resources, and is a great site. I like them, and no i'm not being paid to say this. :)