There are several types of packet software available. Most packet software falls under one of the following categories.
AA4RE BBS is fairly well established, though I am not sure if it being actively maintained anymore. Here is a site with several links to various versions and addons: http://www.bbsdocumentary.com/software/IBM/DOS/AA4RE/
There appears to be a yahoo group for the AA4RE software too. http://groups.yahoo.com/group/4re/
G1NNA has been developed by a couple of Hams over in the UK. Their homepage is here: http://www.gb7esx.co.uk/
Probably one of the more popular BBS packages. F6FBB runs on DOS, Windows as well as Linux. It has been around for awhile and offers message compression when forwarding messages. http://www.f6fbb.org/
This is a unified package to handle both BBS service as well as Net/Rom Node services. This is very popular in Northern Ohio (where the author lives) and across the country.
This is probably one of the oldest packet BBS systems available. There are also neat pictures of W0RLI's house. http://home.comcast.net/~w0rli/
There are several DX Cluster systems available.
G8BPQ is a fairly flexable Net/Rom node system. It can run in the background on a PC for other applications (BBSes, DX Clusters, Chat Nodes) as well as be burned onto an eprom of a Kantronics Data Engine for dedicated node service.
Flexnet is a European implementation of Net/Rom aimed to provide the fastest speed possible. http://www.afthd.tu-darmstadt.de/~flexnet/
One of the more popular standalone packet node software packages that runs natively in a TNC2 clone (MFJ-1270B, 1270C, etc). http://www.qrz.com/download/packet/index.html
Another flavor of TheNet that runs natively in a TNC2 clone (MFJ-1270B, 1270C, etc). http://www.qrz.com/download/packet/index.html
K-Net is a unique option available only for Kantronics based TNCs. It allows you to have a fully functional Kantronics TNC, but also allows full Net/Rom operation. I was unable to find a source for ordering K-Net eproms, but the Kantronics website would probably be a good place to start: http://www.kantronics.com/
This is a fairly popular program to use when connecting to packet from a Windows based PC. http://www.winpack.org.uk/
Written by K4ABT, this is a free program to enjoy keyboard to keyboard, BBS and DX Cluster operations. http://www.buxcomm.com/PKTprimer.html
Commercially written, this software is fairly flexable to get the most out of Kantronics or AEA based TNCs. http://www.interflex.com/
One of the more popular *NOS packages, originally written WG7J and now picked up by VE4KLM, it is widely used today for many applications.
SimpleNOS is written by W0RLI and is also available at his website: http://home.comcast.net/~w0rli/