How about the endowments of major universities? Massachusetts is thinking about doing just that.

WSJ:

Massachusetts legislators, demonstrating a growing resentment against the wealth of elite universities in tight economic times, are studying a plan to levy a 2.5% annual tax on the portion of college endowments that exceed $1 billion.

After all, as Jim Manzi notes:

Viewed purely in terms of economics, Harvard is really a $40 billion tax-free hedge fund with a very large marketing and PR arm called Harvard University that has the job of raising the investment capital and protecting the fund’s preferential tax treatment.

Hat tip to David Nix.