Let's see here.. working your way up from Layer1 for Linux:
- Direwolf sound-card TNC (significantly better than Tom Sailer's soundmodem)
- Ldsped for full AGW API support - DIrewolf doesn't support connection AGW connections.. just unconnected
- Svxlink Echolink server which also comes with the Qtel echolink client - it's also a full blown repeater controller with RTL dongle remote receiver support but you don't have to use all that
- APRX for APRS digi and igate functionality
- Linpac for classic packet client support and lightweight PBBS support
- OpenBCM for nicer light weight PBBS support with a web interface
- UroNode and FPAC (has ROSE support) - for netrom enabled node functionality
- Jnos, LInFBB, BPQ32 for full BBS functionality
I encourage you to check out the various software listings here. For example.. the packet listings: https://radio.linux.org.au/?sectpat=packet
I run the majority of those programs on my home system so I'm happy to give you had if you have questions. Almost all of these programs will need to be compiled from scratch to get the newest versions and as such, I've been writing up a document of how to do this for Centos over the last view years:
http://www.trinityos.com/HAM/CentosDigitalModes/hampacketizing-centos.html
--David KI6ZHD