Many commercial OSS offer paid support or support an "Enterprise" version of the OSS product. Zenoss ([http://www.zenoss.com/]) is a good example of this.
Personally, I see OSS projects that have this business plan as less risky since they are less likely to stagnate due to exhaustion.
See also MySQL and PostgreSQL. They are open source and have free community editions, but they have paid enterprise features and support for those who need more than a database for their Wordpress site.
The donation model is almost never a good idea if your goal is to actually receive income.
Personally, I see OSS projects that have this business plan as less risky since they are less likely to stagnate due to exhaustion.