What to you use as a "fuser -v -n tcp" alternative on Mac OS X

As @vcsjones said in the comments, lsof is the tool for this:

$ sudo lsof -i tcp:80
COMMAND PID   USER   FD   TYPE     DEVICE SIZE/OFF NODE NAME
Safari  804 gordon   16u  IPv4 0x05a2cec8      0t0  TCP 192.168.6.3:50542->stackoverflow.com:http (ESTABLISHED)
httpd   874   root    3u  IPv6 0x05a2a940      0t0  TCP *:http (LISTEN)
httpd   878   _www    3u  IPv6 0x05a2a940      0t0  TCP *:http (LISTEN)

Without the -i, it shows all open files; with just -i it shows network files only; if you specify something after the -i you can restrict by any or all of: IPv4/6, TCP/UDP, hostname or IP, and port number/service name.