Read about unstuffing files on older macs here

My personal favourite for system 6 utilities and system stuff
http://home.earthlink.net/~gamba2/system6.html

Lots of older system 6 compatible programs (shareware and freeware) can be found at:
ftp.jagshouse.com
http://www.jagshouse.com/system_6.html

Very cool Japanese site with pre system 7 software
http://www1.linkclub.or.jp/~kenfuji/

Tons of system 6 compatible programs, inits, hyper card stacks and scientific programs
http://www.mirror.ac.uk/collections/hensa-micros/local/mac/

Enormous archive with 400k disk images full of software for really old macs...
http://www.applefritter.com/software/bcs/index.html

Kids Domain Black and White Shareware
http://www.kidsdomain.com/down/oldmac.html

A huge vault full of interesting old disk utilities (not all of them are system 6 compatible)
http://hyperarchive.lcs.mit.edu/HyperArchive/Archive/disk/

Lots of system 6 compatible stuff can can be found here:
http://files.chatnfiles.com/carousel/Mactosh/

VieuxMac Archive
http://www.multimac.ch/download/