Run netstat -b to determine which programs are listening to which TCP/IP ports. For example, here's the output that I got when I ran netstat -b on my machine. Note: I've replaced the actual digits with X:

Active Connections

  Proto  Local Address          Foreign Address        State           PID
  TCP    DIM3000:XXXX           localhost:XXXX         ESTABLISHED     XXXX
  [firefox.exe]

  TCP    DIM3000:XXXX           localhost:XXXX         ESTABLISHED     XXXX
  [firefox.exe]

  TCP    DIM3000:XXXX           XXX.XXX.X.X:XXXX       ESTABLISHED     XXXX
  c:\windows\system32\WS2_32.dll
  C:\WINDOWS\System32\WINHTTP.dll
  [svchost.exe]

  TCP    DIM3000:XXXX           server2005.re-invent.com:ms-sql-s  ESTABLISHED     XXXX
  [devenv.exe]

  TCP    DIM3000:XXXX           XX.XX.XX.XX:http       ESTABLISHED     XXXX
  [firefox.exe]

  TCP    DIM3000:XXXX           server2005.re-invent.com:ms-sql-s  TIME_WAIT       0

The Firefox browser (firefox.exe) was listening to three ports. Visual Studio 2005 (devenv.exe) was listening to one port.

netstat is useful for troubleshooting SQL Server connection pooling issues, plus it's useful for hunting down spyware on your machine.