ABA Guidelines for the Appointment & Performance of Defense Counsel in Death Penalty Cases (PDF, 178 pages).  
In the form of a  law review article, the ABA Guidelines offer standards pertaining to each stage of the death penalty defense process, from appointment to clemency.

FindLaw
This website provides an overview of the federal habeas process (current through 1999) and a practitioner's checklist for the federal habeas review process after AEDPA.

Death Penalty Information Center 
DPIC offers a comprehensive list of law review articles related to the death penalty arranged by topic, including representing foreign nationals, clemency, state habeas reviews, mitigation, etc.

Death Penalty Defense.com 
This website is an excellent resource for attorneys who have been retained or appointed to represent defendants in capital trials. Access to the website is entirely free, though you must certify that you are a member of a capital defense team to gain access. The site contains strategies, pleadings, and advice for winning capital trials and saving your client from Death Row. The site's content has been written and developed by experienced capital litigator William S. Geimer.

Death Penalty Defense Clinic 
This website has details on the Clarence Darrow Death Penalty Defense College, an annual clinic held at the University of Michigan. It teaches skills and knowledge that are useful in death penalty defense. Instruction includes lectures, demonstrations and, most importantly, small group workshops where participants work on their actual cases. Enrollment is limited to 42 defense lawyers with pending capital cases and the cost is $650.

FEDERAL CAPITAL CASES

Capital Defense Network
This website provides litigation guides for attorneys working on federal capital cases. Guides range cover the authorization process, jury selection, mitigation, and much more.

Federal Judicial Center 
Downloadable PDF of the Resource Guide for Managing Capital Cases (v. 1, Federal Death Penalty Trials).