The autoplug API is interesting, but often impractical. It is static; it cannot deal with dynamic pipelines (insert ref here). What you often want is just an element to stick into a pipeline that will DWIM (Do What I Mean)(ref). Enter the spider.
The spider element is a generalized autoplugging element. At this point (April 2002), it's the best we've got; it can be inserted anywhere within a pipeline to perform caps conversion, if possible. Consider the following gst-launch line:
$ gst-launch filesrc location=my.mp3 ! spider ! osssink
The spider will detect the type of the stream, autoplug it to the osssink's caps, and play the pipeline. It's neat.
Automatically typefinds the incoming stream.
Has request pads on the source side. This means that it can autoplug one source stream into many sink streams. For example, an MPEG1 system stream can have audio as well as video; that pipeline would be represented in gst-launch syntax as
$ gst-launch filesrc location=my.mpeg1 ! spider ! { queue ! osssink } spider.src_%d!
{ queue ! xvideosink }