AMS Digital Mathematics Registry
Hosted by the American Mathematical Society, the aim of AMS Digital Mathematics Registry "is to provide centralized access to collections of digitized publications in the mathematical sciences" and related areas like computer science. This includes both journals and journal-like book series. The resource list can be sorted by title or source (publisher). For every title listed, there's an indication whether it is freely accessible, a subscription is needed, or it is a combination of both.