COW has practice problems and solutions in the following categories:

Calculus I
Calculus II
Calculus III
Linear Algebra
Number Theory
Abstract Algebra

Excerpts from site:

COW is an internet utility for learning and practicing calculus. It was designed at Temple by two members of the Temple University Mathematics Department, Gerardo Mendoza and Dan Reich.

The principal purpose of COW is to provide you, the student or interested user, with the opportunity to learn and practice problems in calculus (and in the future other topics in mathematics) in a friendly environment via the internet. The most important feature of the COW is that you get to know whether your answer is correct almost immediately. It is as if you had a tutor looking over your shoulder and helping you along as you work. This will be true no matter where you are or what computer you use, as long as it is connected to the internet and has a web browser.

The student component of COW (called the Manager) generates calculus examples and exercises in "modules" for studying, tutoring and practice. A number of the modules allow you to experiment by letting you change values or parameters in a function or graph and then see the effect. These modules are called "hands on" modules, and are marked with an asterisk. The component of the COW accessible by instructors (called the Reporter) handles assignment and automatic grading of homework, reporting on student work and class management.