Sandbox for editing
This page is about a place to test and play around with making OpenStreetMap edits. If, instead, you are looking for a Wiki sandbox (a page where you can play around with wiki editing), go to Sandbox.
You can use the development server for testing OSM editing. Here, you can try out complex things before doing them on the OSM database. If you are developing editing clients and automated edit scripts, this is very useful (and, in fact, required, or at least strongly recommended. See Automated Edits code of conduct).
The dev.openstreetmap.org server hosts several versions of the OSM APIs for use by anyone for testing clients against, or as a data sandbox. The API versions are detailed here:
Most of the time, for testing clients or as a sandbox, http://master.apis.dev.openstreetmap.org/ (commonly known by its alias, http://api06.dev.openstreetmap.org) is the one you want, as that is (usually) the same code as the live site. The others are various development branches where new code is being tested.
You need to 'sign up' to these sites as if you were a fresh new OpenStreetMap user. Once you have done so, you should configure your client with the correct URL address. For example, in JOSM, you do it in the following menu: Edit > Preferences > Connection settings. Uncheck "Use the default OSM server" then, for example, as the "OSM Server URL", write: "http://api06.dev.openstreetmap.org/api" and add your authentication details.
These sandboxes are installed as instances of The Rails Port OpenStreetMap rails app which also includes a copy of the openstreetmap.org website and things like user profiles and diary entries, but the database is mostly empty (just populated with some test data). There's no default renderer for them, so in order to see the results on a map you'll need to extract the data and render it yourself (using whatever map style you like) - see http://wiki.openstreetmap.org/wiki/Rendering .
Other instances can be found running elsewhere, sometimes as a sandbox for playing around, sometimes as mapping database with a particular purpose separate from OSM. You can set this up on your own server too. Developers can also get account on the OSM development server for this kind of experiment.