Where can I get XYZ software?

Depends on your distribution. Some distributions host large archives of all sorts of goodies in the appropriate format for that distribution.

• Redhat has several sites worth looking at: ftp://ftp.redhat.com/ contains the main distribution itself, so some packages will be found in here; ftp://updates.redhat.com/ contains official redhat updates; ftp://contrib.redhat.com/ contains packages created/maintained by parties external to Redhat, and have been contributed for general consumption.

  RPMs are not a uniquely redhat thing. Several other distributions such as Redhat, Caldera Openlinux and Suse also use RPM. Most RPMs are distribution interchangeable. http://rpmfind.net is a non-distribution specific repository of RPMs, and it is worth taking a look there also.

• Debian has ftp://ftp.debian.org, to which dselectet al. will fetch packages in that uniquely debian way.
• If it's not available as a .deb or .rpm, it is guaranteed to be available in the format that slackware calls a package -- .tar.gz. ftp://metalab.unc.edu os a great place to find all sorts of tarballs.

  Try also searching at FAST FTP Search and linux software search.

  Another page you can use is Red Hat's Linux Apps and Utilities page.

  A good place to check out (daily) for software updates is Freshmeat.