Echo's what CouchDB makes available to it's Request Object (which it in turn gives to _show, _list, and _update functions) when receiving an HTTP request.
Pretty handy for getting started with CouchApps!
Grab couchapp.py, then...
$ couchapp push . http://{user}:{pass}@localhost:5984/couchdb-http-echo
$ curl http://localhost:5984/couchdb-http-echo/_design/http-echo/_show/echo
$ curl http://localhost:5984/couchdb-http-echo/_design/http-echo/_list/echo/all
$ curl -X PUT http://localhost:5984/couchdb-http-echo/_design/http-echo/_update/echo
Public Domain (srsly! this is hardly copy-writable)