It depends on whether you want a self-contained client with built-in
UI, or a language binding API.
The only Open-source version of the former is an early and as yet
incomplete prototype for Node.js (https://github.com/arturadib/
simpledb-webview) but it would need a bit of modification so that it
could be pointed at an M/DB endpoint URL
I've attempted to bring M/DB to the attention of the SDB Explorer guys
on a number of occasions, but I have been ignored by them. Perhaps
some user pressure may convince them that M/DB compatibility would be
worthwhile. It would be a negligible amount of work for them to allow
it to point to a different endpoint URL, after which it will think
it's still talking to SimpleDB.
There are numerous Open Source language bindings for SimpleDB. Many
already work with M/DB (eg boto for Python) or can be easily modified
to accept a different endpoint URL (eg the AWS clients for .Net and