Tax Lien Investing/Alternative Investments

Has anyone heard about this form of investing? It sounds similar to investing in discounted notes. Basically the overview I've gathered is that local county governments need money, and they are willing to sell their lien certificates at a considerable discount. Their is an auction you bid on the interest rate(bidding down) like say the max rate in Arizona is 16%, the bid would start at 16% to 15.5% all the way down to minumum interest someone is willing to reap. The owner of the property pays the face value of the certificate. You could defintely reap above average returns on your investment. Iowa statutory limit is in the 20% from what I've read. Obviously it comes with risk, but you certificate is backed by real estate. It seems to work better than mortgage notes, because interest rates aren't going to reach 20%.

Is anyone familiar with this type of investing?

Also, what are some other alternative investment, thats out there?