The logs themselves and the various tcpdump options provide you with valuable tools to gain insight into what happens in your network. Not surprisingly, other tools have been developed to operate on PF log data, collect statistics and do various forms of graphing.
Of special note is Damien Miller's pfflowd, which collects PF log data, converts to Cisco NetFlow™ for further processing. Damien also develops and maintains see flowd for NetFlow™ collecting purposes. This Cisco originated data format is supported by a number of different products, and the ability to generate data in this format may be important in certain environments.
One other log data application which is well worth noting is Daniel Hartmeier's pfstat, which collects statistics from PF logs and generates graphs from the data. It's a fairly flexible package which takes a lot of the heavy lifting out of presenting log data.