Its common to see questions relating to finding discounts on wine tasting and other aspects of a wine country adventure, so here are a few resources to try. 

For Coupons:

The Individual winery websites are the first place to look.  Then expand to the regional tourism sites like wineroad.com, russianrivertravel.com, bestinsonoma.com, et al. Then expand to the sonomacounty.com website which promotes the entire county. There are always going to be some wineries that you can't find in these sites, and they are the usually the ones that are not open to the public and/ or are very small.

Publications:

When you arrive in the area you are likely to find numerous opportunities to pick up the tourist-oriented free publications like Wine Country This Week, The Preiser Key, et al, at hotels and restaurants. These are readily available and may advertise special deals. Some of these discounts require that you mention the ad ( or publication).The numerous Visitor Centers are wonderful resources for similar guides, general information, tasting coupons and maps. Some of the wineries have  advertiser racks with flyers for different activities, and some of these are tasting coupons for wineries in the region.  Tthis seems especially prevalent in the Alexander Valley/ Healdsburg area.

Scuttlebutt

Talking to your servers can be very helpful.  Sonoma winemakers work together to promote their product, so sharing is common.  Friendly staffers promoting other wineries in their region may have coupons to offer.. Talk to your server and ask about wineries in the area that might match your palate, or that may have a particular wine of interest to you...its a good way to get helpful suggestions and can lead to a discounted tasting because of the referral.

Special programs

ASide from the big events like Passport weekend, etc,  The Wine Road organization has the Ticket to The Wine Road program, which you can investigate on the wineroad.com site.  This program includes not only wine tasting but some lodging and dining discounts as well.

 They have also partnered with the Visa credit company to provide the Visa Signature program which is a huge boost to the savings if you remember to use it.   This program discounts  wine tastings and purchases at participating facilities...there were around 70 last fall.

Free Tastings

Yes, they do still exist, but finding them takes some work. There is a regularly updated list kept on the California Winery Advisor website.  Other sites have lists too, but this seems the most accurate over the last 5 years.