(with only one 's') so a google for Ingres gets approx 620K hits.
My DBD::Ingres Module (as jZed noted) is the DBI-Driver for Ingres.
Ingres has recently been Open-Sourced (more or less: the source is available, but CA reserves the rigth to commit changes) and is freely available at http://opensource.ca.com/projects/ingres. It's a fully-fledged RDBMS with transactions, locking, recovery and so on. It's been around for nearly ever - my first contact with it was in 1989 when it went from version 5.x to 6.0. At that point it was one of the three major RDBMSs (Oracle and Informix where the other two). Later is has languished after being bought by first ASK and the CA. CA have now thrown a lot of effort into it - they found out that it was silly to have to pay Oracle a license for the embedded DB in their applications, when they had a fine system in-house. They have added a lot of Oracle compatibility in the last couple of releases, and the OSing of it is hoped to add more users, hopefully in time giving more sales and support.