In present times when companies are looking to add projects and cut costs open source is a very viable option. In my experience more than 75% of the SQL Server projects would work well on MySQL. At an internet company I worked at we used MySQL for over 300GB of data. There are great analysis tools for it and lots of community driven support.

Where I do see a vacuum in the open source world as it relates to databases is ETL & multidimensional databases. There are some recent projects in this area of BI, but they are not close to as mature as the commercial offerings from companies like Business Objects or Oracle.

I would also recommend sticking with InnoDB for enterprise projects because of transactional support, performance and supportability of large databases.