I am seeking an exhaustive list of charities in which Martin Sheen has
shown a significant interest in the last ten years.  Organizations
that have received significant contributions of time and money over
multiple years are of particular interest.  Also the names of 501c3's
on whose board he currently sits or recently served is also sought.

Some of the resources mentioned here might be of use to you:

http://www.answers.google.com/answers/threadview?id=398603

Henryjust,
As you will be aware Mr Sheen?s interests extend across many fields.
He lends his name to a wide variety of causes, but as I understand
your question, casual association may be excluded, as can political
affiliation. The following remain (all are 501c3?s):

1983
Office of the Americas
Co-Founder & Board Member since at least 1998
http://officeoftheamericas.org

1984
The San Carlos Foundation
Board Member (Co-founder) & Vice President since 2000
http://sancarlos.nonprofitoffice.com

1992
L A Family Housing
?LAFH has enjoyed the support of Mr. Sheen since 1992?
(http://lafh.org/3events/award2001.htm)
http://lafh.org/index_flash.htm

Date of Board Membership is uncertain but is current
Sea Shepherd Conservation Society 
Media and Arts Advisory Board Member
http://seashepherd.org

For an excellent article on Mr Sheens philosophy regarding social
activism see here:
http://www.progressive.org/july03/intv0703.html

For an extensive library (articles/photos) see here:
http://execprivilege.tripod.com/index.html

Just for laughs (but for a good cause) see here:
http://www.clothesoffourback.org/2004auction/47.html

The Internal Revenue Service (IRS) provides a list of commonly asked
questions relating to the disclosure requirements of organisations
that are tax exempt, such as charities (or 501c3?s). Question 6, asks
?are tax-exempt organisations required to disclose the names or
addresses of its contributors??. See here:
http://www.irs.gov/charities/article/0,,id=96430,00.html#1

The answer given is no and while the information may be obtainable,
this commenter chooses not to do so.

Kind regards,
gfmaster