What program is good for designing a poster?

I'do go for inkscape

It's vector-based and i've used it quite a few times. I think it's a very good option.

It's help is also very good, as it has many tutorials in order to learn how to use it (you can access them through: Help > Tutorials)

Well that really depends on your preference; If you like to use vector software such as inkscape which will allow to design like if you were to be using Adobe illustrator. Then the other route would be to use a raster based software such as the GIMP which allows you to design like if you were to be using Adobe Photoshop.

Vector = Lines and Curves

Scales better if you want to later on resize your image but only works well when creating original works without any photos.

Raster = Dots per inch or DPI

Mostly used for modifying photos or creating digital art work with photos. This works great if your artwork will make use of heavy photo editing/manipulation.

Note: In inkscape you can import raster images and add some effects and export your designs as raster images with the .png file extension. inkscape can also export or save your designs as a PDF file.

I would choose OpenOffice.org/LibreOffice Draw. LibreOffice is installed by default in 11.04, before that it was OpenOffice.org.