Why can't I traceroute or access certain websites in my browser even though I can ping them from terminal?

Solution 1:

Problem solved! It looks like one of my login items was interfering. I cleared out several applications that I didn't really need to have load at startup, and the issue seems to have cleared up since. Unfortunately, I don't know which application was interfering, and I'm not sure I'll take the time to go back and pinpoint it anytime soon. Have already poured too much time into this. Thanks though to harrymc, mehaase, and Spiff for your help!

Solution 2:

PeerGuardian installs itself as a service, so even if the GUI is not running, it still is active and may block some sites.