You might want to consider something like osTicket, which is free: http://www.osticket.com If you need something heftier, there is Kayako eSupport: http://www.kayako.com
If you just need something simple, you may want to consider have a custom one used. I've used OSticket with success, but I prefer Kayako, though it's a paid software. Also, have you considered any live chat softwares? Your question was pretty vague.