MiniExiftool version 0.4.0 released! Second try. 
http://miniexiftool.rubyforge.org/
MiniExiftool is a wrapper library for the Exiftool command-line
application (www.sno.phy.queensu.ca/~phil/exiftool/) written by Phil
Harvay. One of the completest tool for reading and writing meta
informations (EXIF, IPTC, XMP) on image files which supports many file
formats including raw formats.
Installation:
gem install mini_exiftool
or alternative download the tarball and run
ruby setup config
ruby setup setup
sudo ruby setup install
Tutorial:
http://miniexiftool.rubyforge.org/files/Tutorial.html
Changes:
- MiniExiftool::Error inherits now from StandardError
- Alternative installation via setup.rb
- Bugfix
  Saving of non-readed tags doesn't work with tags with hyphen
  Thanks to Robin Romahn for reporting the bug
- New methods: MiniExiftool.all_tags and MiniExiftool.original_tag
- Original tag names (all and writable) are now saved via pstore in a
  file for better performance