Clicksor is easily the current leader of the small publisher Adsense competition. They have payouts up to a market leading 85% and unlike a number of alternatives the cost per click bid values are high enough that you can earn a decent income. Their terms and conditions suggest that you should only place one copy of their code on a page but as long as you only place a single pop-up or DHTML code on a page they seem happy to let you place many context sensitive ad blocks on a single site. Clicksor also has a big advantage over a number of the lower ranked alternatives on this site in that their ads are truly context sensitive. Thus while you can provide keywords they have technology that will also show ads targeted to the content of your site. This means your visitors are exposed to ads of the same subject as they were searching for when they reached your site and thus vastly increases the click thru ratio and thus your sites earning potential. The context sensitive nature also saves you the hassle of having to specify keywords for all of your sites and decide in advance what people might be looking for. The payment schedule with Clicksor is also much better geared to the small site owner. In any pay period in which your earnings exceed $50.00 ($20.00 with PayPal), payments are made either by check or instantly through PayPal ($20.00 minimum for PayPal). Revenue totaling less that $50 (PP $20) will rolled into the following period. The pay period is based on a 15 day schedule so you can get paid much more frequently. Clicksor also do not require a Social Security (SSN) or Individual Tax Identity Number (ITIN) to join and so while this 'technically' does not help US site owners with their tax liability it does make it much easier for non US citizens to join up and start earning revenue.
I'm doing them both it depend on the website structure and niche, sometimes Clicksor is better sometimes AdSense and sometimes neither.
Clicksor only accepts high-traffic sites. Although I had it on my site previously, the payout is quite disappointing. I still prefer Bidvertiser as an Adsense alternative.