I have done a lot of this for the past 3 months; I'm not 100% sure of its effectiveness, but I KNOW that it does make a difference.
I search through groups.yahoo.com and other listservs looking for organizations that are likely to be Kerry-friendly in my state or hometown, and I post information that I think has the potential to be useful.
For people who want to get active immediately, I offer links and phone numbers to the most politically active organizations in the area. For me, that's the local Dem HQ, but for others, it may be www.actforvictory.com or www.moveonpac.org. I also offer the Kerry site, www.johnkerry.com/volunteernow.
When the Kerry/Edwards folks come to town, I e-mail announcements of how to get tickets to friends, family and other Kerry supporters.
For people who are undecided, I post non-partisan websites such as www.presidentmatch.com and
http://www.msnbc.msn.com/id/5993610/ to help them find out where the candidates really stand on issues and what their similarities and differences are without all the propaganda and attacks. I also recommend www.vote-smart.org.
I keep my ear to the ground about campaign activities in my area and make sure they are posted on various websites, especially volunteer.johnkerry.com. I occasionally organize them, but my primary interest is to make sure that folks tied into the website know about local activities. I also visit www.acthere.org and www.democracyforamerica.com and pull in every listed person who lives near me as a "friend", and regularly e-mail them information about things happening in the area that they can get involved in.
Last... I buy stuff and distribute it. Bumperstickers, buttons, yardsigns. Most of what I've bought, I have shared freely. The signs I am "selling" because they're relatively expensive. I donate the profits or extras to the local Dem HQ.
While I can't say that I'm impacting hundreds of thousands of people, I DO know that my work makes a difference, because people respond and say thank you.