I believe that Spring is unique, for several reasons:

It addresses important areas that many other popular frameworks don't. Spring focuses around providing a way to manage your business objects. Spring is both comprehensive and modular.

    Spring has a layered
    architecture, meaning that you can
    choose to use just about any part of
    it in isolation, yet its
    architecture is internally
    consistent. So you get maximum value
    from your learning curve. You might
    choose to use Spring only to
    simplify use of JDBC, for example,
    or you might choose to use Spring to
    manage all your business objects.
     And it's easy to introduce Spring
    incrementally into existing
    projects. Spring is designed from
    the ground up to help you write code
    that's easy to test. Spring is an
    ideal framework for test driven
    projects. 

Spring is an increasingly
    important integration technology,
    its role recognized by several large
    vendors.