I love this thread !
I've used a L600r with 512MB RAM P3-600Mhz 100Mhz FSB for several years, running the latest FreeBSD -nogui strictly used for cisco switch administration. It's lightening fast and secure, living on a subnet not associated with any other local network.
You can use these older machines for a plethora of things, including file servers, local git repo, firewalls, ect...
Not sure about throughput though, I can't imagine it would be all that great. 10GB HDD w/ 10/100 card, it might though.
Either way, kudos to you for using 'old' hardware and getting great use out of it