Don't consider this the 'definitive' answer, but here's my 2 cents:

Mandrake 9.0 -- extremely easy install and use
Slackware 8.1 -- more for the linux power-user
Red Hat 8.0 -- very good support, large user base, lots of distro-specific linux apps are written for it
Debian 3.0r1 -- giant distro (7cds), large user base
Knoppix 3.1 -- good bootable cd distro, great if you want it copied to the hd
OpenBSD -- not really linux, but based on the BSD kernel. outstanding security, performance... amazing for webservers
FreeBSD -- similar to OpenBSD, has arguably the fastest TCP/IP stack

All of the isos for these can be found on www.linuxiso.org except for the OpenBSD one -- they usually sell the cds but you can find unofficial (still legal) cd images via google.