Who's Paying for all of These Political Ads, Anyway?

Have you noticed there are a lot of political ads running these days? Of course you have, especially if you're in one of the "hot race" areas of the country like I am in Las Vegas.  A lot of the ads end or begin with "I'm So-and-so, and I approve this message." But this year there are also a lot of ads sponsored by patriotic-sounding groups you've never heard of, like Citizens for Lower Taxes and Freedomy Liberty!  But these groups can often be shadowy and mysterious.  Who are these people?  It would seem that they a) don't really want to know us who they are, and b) are effectively trying to buy elections all over the country.