How to obtain a bounding box for use with OSM tools

There are a few sites out there (e.g. http://osm.duschmarke.de/bbox.html and http://bboxfinder.com/) that allow the selection and display of a bounding box.

The main OSM site itself used to do this (see trac here), but no longer does - and since routing went live, the last OSM dev site that supported it has gone away.

What I'm looking for is:

• Something to create a bounding box, that works across platforms and across window managers
• Something to display a bounding box in the "usual" OSM format (as used by the API etc.)

It'd be nice if it also allowed the display of different tiles too - either layers available on osm.org or elsewhere.