Among the Amazon Web Services or AWS if you prefer acronyms, SimpleDB is one of the less popular.
I've been using this service it for a while in some personal projects using the aws-sdb, so far the only choice to deal with SimpleDB at a low level.
Since I wanted to overcome some of the limits of aws-sdb I wrote a new gem aws_sdb_bare.
Main feature of the gem are:
* Complete implementation of SimpleDB api
* XML parsing uses Hpricot or Nokogiri
And be free of chosing your HTTP connection library and strategy!
You can use Net::HTTP or curb or go concurrent using eventmachine or typhoeus to manage your connections with sdb
So just install it
sudo gem install hungryblank-aws_sdb_bare -s http://gems.github.com
And use the code in this gist to get started, rdoc are available.
For a more complete overview of the SimpleDB ruby libraries currently available you should read this article