What type of natural hazards do you investigate? Pleas be carefull, hazard assessment needs a lot of experience, information on historical events as well as knowledge of the specific terrain. I do not know any usefull open source software. For Alpine natural hazards (debris flows and snow avalanches) we developed RAMMS ( http://ramms.slf.ch). But this is no open source tool.
Try ILWIS software provide many tools for Mapping the terrain/causative parameters and integrate the factors using various model for hazard and risk assessment. First decide your model then create database finally integrate.
The Delisting Risk Assessment Software (DRAS) is a user-friendly, stand-alone software program that calculates the potential risks associated with disposing a given waste stream to a landfill or surface impoundment. For a given waste stream, the DRAS calculates both the waste's aggregate risks and also back-calculates each waste constituent's maximum allowable concentration permissible for delisting. DRAS requires the user to assign a target cancer risk and hazard index.
For Earthquake hazards, I would recommend HAZUS. It is free but you have to have license for ArcGIS 9.3. However, it needs comprehensive database for building, infrastructures and socio-economic aspects. As it is data demanding software, you should be very cautious before using it
Integrated Land and Water Information System (ILWIS) is a very good software for your purposes and it's free. Of course if you can find a license for HAZUS, i think that it is better. It depends on what are you looking for.
Regarding Landslide and flood risk, my office is developing specific tools that can maybe be adapted to your needs; if you go to the web site (www.cfumbria.it) you can find more info (e.g. LANDWARN for landslide early warning systems). Anyway those are focused on EW purposes..
You're right unfortunately so far only a small part, a brief description of the early warning system for landslides is in English. I send you the above document and other info at your mail, and as soon as my collegue finish to translate a doc for flood prediction i will send that to you