Knocker is a simple port knocker written in Python. To configure it, make a ~/.knocker:

[config]
host=my.awesome.server.com
port=22
command=ssh %host
openseq=1234,5678,9123
closeseq=1111,2222,3333

Then, just run knocker. You can also run knocker.py -h to find out how to configure it purely via the command-line.