Full disclosure: I work for the Sonoma County Tourism Bureau, so your post caught my eye (I was looking for SF restaurants...)
Here's some good info:
The Map Store in Windsor has a cool interactive wine map. I don't know if it shows if tastings are free or not: http://www.themapstore.com/thewinemap/
Barrel Tasting is all of March, and a lot of the fun is in the Russian River area:
A few freebies in the RR area (on or near Westside Road) are:
Korbel Champagne Cellars
Porter Creek Vineyards
Davis Bynum Winery
Hop Kiln Winery
As a general note, Sonoma County has many free tasting rooms, or rooms that will apply a small tasting fee to the purchase of their wine.
And, shameless plug, if you want more info about things to see and do up here, please check out our website: www.sonomacounty.com.
Now, I need to find some places to go eat in the City....
Actually, a lot of Sonoma County is free. Maybe not Sonoma Valley, but I haven't been through there in a while.
Alderbrook, at the north end of RRV, charges, as does J Wine (but at least you get food there). Westside Road is mostly free I believe. Rochioli may be charging. South of River Road, I think Lynmar charges, but not sure who else.
SFGate.com's Wine section (web version of SF Chronicle Wine section) has a "Wine Country Guide" search section near the top of the front page (right side): www.sfgate.com/wine
This "Wine Country Guide" allows you to narrow your search by selecting "Special Features" from a drop-down list, one of which is "Complimentary Tastings." The other drop-down list is for "Locations," but unfortunately Russian River is not among the specific areas on that list. I guess it would be under "Other Sonoma County."
Note that the search results give very brief descriptions for each winery, but click on the wineries' names for more details.