Do I have to have a special router or modem to enable VOIP?

"No, you can use a VoIP application (e.g. Skype, VoipStunt, VoipBuster, etc.) from your computer without a 'special router or modem'.

Unless you want to use your regular telephone, in which case you'll need a SIP gateway."

While any old router should work fine, depending on your traffic volume and type, it may be helpful to have a router that supports QoS, to prioritize VoIP traffic as highest priority.