There are a lot of alternatives to OpenX but there is no one answer that fits everyone. If you're looking for a free alternative then you'll find plenty of options but I've found that you get what you pay for in terms of support and features.
If you're serving more than 100 million impressions each month and you need a dependable ad server then the list is pretty standard: DoubleClick, 24/7 OAS, AdTech and Adzerk. With the exception of Adzerk many of these ad servers require a contract and large monthly minimum to get started.
If you're serving between 1M and 100M then I think DoubleClick Small Business and Adzerk are good options since they are easy to signup for and don't require a contract. Adzerk has more support and is easier to use but DoubleClick is the bigger name. Also note that this site (StackOverflow.com) uses Adzerk.
Full Disclosure: I work for Adzerk so feel free to reach out if you'd like more information.