A more complete and updated list may be found on the “Links”; section of the LPI web site.
The Linux Professional Institute: http://www.lpi.org/
Articles relating to certification:
http://www.linuxgazette.com/issue33/york.html
http://www.linuxgazette.com/issue34/york.html
http://www.linuxgazette.com/issue35/york.html
http://www.linuxgazette.com/issue37/york.html
Linux Training Resources: http://www.linuxtraining.org/
Linux Journal Forum: http://www.linuxjournal.com/forum
Other Linux certification programs:
http://www.linuxcertification.org/
http://www.digitalmetrics.com/
http://www.redhat.com/