I've seen this problem a lot and I still haven't found a solution for it yet.
I can connect to my website via localhost and 192.168.0.6, but when I try to access via my external IP from www.whatismyip.com, it comes up with a page saying unable to connect.
I port forwarded port 80 in my router and my firewall is not blocking it. I know this because I disabled my firewall and it still didn't work.
I'm pretty sure I port forwarded it right. Went to the Gateway tab and selected Forwarding. I entered my IP (192.168.0.6) and put 80 for both starting and ending port. For the TCP/UDP choice, I selected Both.
My laptop is running Windows 7 Ultimate SP1 64bit.
My router information:
Cable Modem Information
Cable Modem : DOCSIS 2.0 Compliant
MAC Address : 90:6E:BB:54:60:2A
Serial Number : A9B0716901440
Boot Code Version : 2.1.7d
Software Version : 6.23.1107
Hardware Version : 2.33
CA Key : Installed
MTA Information
MAC Address : 90:6E:BB:54:60:2C
CA Key : Installed