What are the best internet sites and/or books on how parents
should talk with teenagers (13-17 boys and girls) about sex?

gogansa..

As the father of three kids, one of them a young teen, I am as
concerned as you about the best way to broach sensitive subjects.  My
opinion is that we, as parents, have a responsibility to talk with our
kids and provide them with the information that we feel will help them
in difficult situations.

Here are some great resources to help parents discuss sex with our
children:

Talking With Kids
http://www.talkingwithkids.org/
A great collection of tips and resources about discussing tough issues
with kids.

Campaign for Children, Inc
http://www.cfoc.org/4_parent/4_kids.cfm
There is a recommended reading list here that has several great
publications that can help
http://www.cfoc.org/4_parent/4_recommend_reading.cfm

Idiot's Guide to Talking to Your Kids
http://www.idiotsguides.com/Quick-Guides/MG_Talk_Kids_Sex/file.htm
Yes, from the makers of those famous yellow books, a very nice
resource with some great tips

A Healthy Me!
http://www.ahealthyme.com/topic/sextalk12to16
Another great resource that focuses on kids aged 12-16

iVillage
http://www.parentsplace.com/features/sexed/articles/0,10335,258709_115820,00.html
Scroll down the page to pre-teens and teens and there are some
articles on various subjects

I wish you luck on this endeavor in helping your kids.  Being a parent
is a tough job and I hope the information provided above helps make
this important step a little easier.  If you need any further
clarification, please let me know.

Regards,

-THV

Search Strategy:
talk to your kids about sex

great answer, provided some great internet sites that were hard to
find.  well worth it!

The following resource, part of the Open Directory Project, includes
links to websites aimed at teenagers on the subject of sexuality:
http://dmoz.org/Kids_and_Teens/Teen_Life/Teen_Sexuality/

- Sites that are flagged for "Teens" are directed at those aged 13-15
years.

- Sites that are flagged "Mature Teens" are directed at those aged
16-18 years.

- If both tags are displayed, the sites are directed at both groups.

These links have been hand-edited and tagged by volunteer editors, who
have reviewed them for any age-inappropriate content. While it is
possible that some sites have changed since they were reviewed and
indexed, this resource can be generally regarded as kid-safe. You may
find this to be a suitable resource for your kids, so they can follow
up on your discussions or search for answers to questions they may be
too embarrassed to ask a parent.

Incidentally, the same directory is available through Google, but the
age tags are omitted:
http://directory.google.com/Top/Kids_and_Teens/Teen_Life/Teen_Sexuality/

Thank you expertlaw-ga for your excellent referral to
http://dmoz.org/Kids_and_Teens/Teen_Life/Teen_Sexuality/ 
This is definitely an extremely useful site.