I would ignore the tax assessment completely, as they're typically lagging. I would then suggest using Zillow and Redfin (or your local equivalent) and averaging the two values.
Redfin tends to undervalue our primary home and overvalue our rentals, which are in a completely different market. Zillow does the reverse. Yet, when I add them up, the total value is almost to the dollar, which cracks me up.
Another frugal real estate weirdo way to have fun is to use either site to do a search. Set up parameters that match your home for lot size, square footage, bed/bath count, etc. Choose "Recent Sales" and see what comes up. Average those, adjust for different amenities and you've got a decent idea of current market value.
Recently, we did this with our primary. We have a huge, 3-car, 1050 sf garage. They're not all that common in our area, so I only got 6 matches, and the prices weren't great. I repeated the search, without the 3-car garage, and got 21 hits and much higher values. Are houses with smaller garages worth more? No. It's just that all the houses with three car garages that have sold recently were older/smaller/whatever. That's how these estimates can get so wonky.
