The Scandroid
The Scandroid is a program that scans English verse in iambic and anapestic meters. You can load the text file of a poem, or type lines in by hand. As you "Step" through the process, the program explains what it's doing: identifying syllables and lexical stresses, dividing the line into feet and so on. This makes it a self-teaching tool. At the same time, it's suitable for some kinds of research on metrics. Though it uses the traditional foot-based scansion, some of its techniques and principles of construction derive from recent decades' work in generative phonology. There's a Manual that describes both how the program works and why it works that way.
I don't know of another program that does quite the same thing. It's fairly successful; it handles Larkin's "Church Going" and Browning's "My Last Duchess" and Carroll's "The Hunting of the Snark" pretty well, and none of them is particularly easy to scan. Its treatment of a word (syllabification and stress) can be changed by the user. It employs two different algorithms for dividing iambic lines into feet, and another for anapestics; when it loads a poem-file it samples the lines to deduce whether to treat them as iambic or anapestic.