EFF used to have a much more extensive page at this site.  Other search services do a better job.  In lieu of that we are providing some of the basic links so that you can better do your own research.   This will be limited to a few U.S. links and a couple of search engines.

U.S. Senate:   http://www.senate.gov/
From this site you can find all senators and e-mail them directly.

U.S.  House of Representatives:   http://www.house.gov/
From this site you can find all congresspeople and e-mail them directly.

White House:    http://www.whitehouse.gov
This is a handy page and has links to other parts of the executive branch.

The Congressional Internet Caucus:   http://www.netcaucus.org   Those in Congress who should have the most interest and knowlege of the net.

To write a senator or representative you can go to the EFF web page:   http://www.eff.org/congress/ where you can generate e-mails directly or find out who is your representative if you know your address and zip code.

Govbot The University of Massachusetts has a service which lets you search many government data bases:   http://ciir2.cs.umass.edu/Govbot. This has a fairly sophisticated search engine and can be very helpful.

Yahoo!'s Government Web Directories: http://dir.yahoo.com/Government/Web_Directories/

In addition to the above sites, almost anything else you would like to find, including information on how to get in touch with a foreign government's web site is available from any good search engine.