In the last few weeks we merged the work from the QGIS SEXTANTE project into the official QGIS repository's master branch. This means basically that in QGIS 2.0 SEXTANTE will be a default plugin, available immediately on installation of QGIS. SEXTANTE uses a pluggable architecture, providing front ends to SAGA, GRASS, Orpheo Toolbox, MMQGIS and other useful tools. If you are familiar with ArcGIS then thing 'Arc Toolbox' here and you should have a notion of what SEXTANTE will do for you. If you are running Ubuntu 12.04 it is pretty easy to install the needed backends to fully benefit from SEXTANTE using these few commands:
sudo add-apt-repository ppa:otb/orfeotoolbox-stable-ubuntugis
sudo apt-get update sudo apt-get install python-saga python-otb grass saga otb-bin otb-wrapping-python otbapp
If you open QGIS now, all the SAGA and OTB tools should be available to you.
Note: you should use a recent GDAL from the ubuntugis-unstable ppa to avoid GTiff issues.