After installing, trying and discarding typus, casein as well as ActiveScaffold, I ended up with one of the best plugins I've seen, and certainly the best for utterly effortless admin panel setup, admin_data.
- super-quick to install (Rails 3 is the gem, Rails 2 is a plugin branch, no digging through trees on github),
- unintrusive (all code is in vendor/admin_data where it belongs),
- requires no set-up,
- correctly (!) gets all model information from your model definitions (primary_key, foreign_key, relationships etc.),
- including multiple databases, SQL Server connections via activerecord-sqlserver-adapter, and even
- composite primary keys,
- making it great for legacy data,
- uses your existing authentication solution.
It's basically completely abstracted sitting on the top of ActiveRecord, and this design choice is very powerful. Not only that, it even checks your model definitions along the way. If your table is visible and editable in admin_data, it's probably defined correctly (especially with regards to foreign_key, primary_key(s), set_table_name and other stuff relevant for legacy data). And did I mention the configuration is one block in one file in your app?
Have a look at the readme whose four lines are indicative of the DRYness and brilliance of this plugin.