Xmp Manager is a GUI to manage XMP metadatas on GNU/Linux.

Currently XmpManager is a very experimental software and it could not work as expected. The features implemented are minimal.
Why should I use metadatas?
Metadatas permit to attach more info to your files, ex. description, author, coordinates.. This datas are embedded in the image file, so you won't lose them even if you'll upload the image on a website or you will burn this image on a DVD. 

Here a screencast.

Why should I use XMP?

• XMP is an extensible schema (proposed by Adobe) so your EXIF, IPTC, JFIF,.. metadatas could still live XMP. the main advantage is that it is extensible and xml based so if you need more fields you haven't reinvent the wheel, or another incompatible standard, but just a new schema.
• XMP is the embedded preferred format for the Creative Common Licence. (in italiano)
• XMP is filetype-indipendent, it supports JPEG, PSD, TIFF, AVI, WAV, MPEG, MP3, MOV (via XMP Toolkit version 4.0)

Features

It support 'title', 'description', 'creator' fields (but It is very easy to add new fields)
It writes metadatas on-the-fly
It support multiple image editing

Installation

You can download latest Xmp Manager on Github
Xmp Manager 0.4
Xmp Manager 0.3
Xmp Manager 0.2
Xmp Manager 0.1

There isn't an installation yet, you need ruby-gnome2, exiftool and nautilus-actions(optional but very raccomanded).

For Ubuntu Linux

sudo apt-get install libimage-exiftool-perl ruby-gnome2 nautilus-actions

An set Nautilus-actions similar to this:

Ideas for the future

There are many things that is possible to do, the priority is a complete xmp fields support and then the selection of the tags(keywords) in the folder with checkboxes.

Embedded geotagging via Google Maps is for 1.0

Bugs

• If the image path has a whitespace ' ' it doesn't load the image
• .. but now we're just brainstorming, It's not a priority now

Notes about metadatas around us

Xmp implementations
Windows Vista is among us, even if I don't plan to migrate to Vista, but Windows Vista Photo Gallery is very well done and probably is the only killer app I see (The unique thing that Beryl can't do :D).
F-Spot and other OSS tools still don't implement XMP in a user friendly way.

Xmp and Flickr
Flick supports import EXIF, but it doesn't export anything so your geotagged photos with coords and description will be locked in Flickr forever, think about it!