Hi everyone,
Kind of a general question here.
Does anyone knows of any open source app for quickly previewing 3D GEO obj files?

Nico.

Not open source, but if you have houdini installed, gplay is a tool that can open any geometry file houdini supports.

meshlab?

http://www.meshlab.net/ [www.meshlab.net]

It pains me to say it but Autodesk's FBX Review is very slick. It uses Maya's Viewport 2.0, works with all the main formats - FBX, OBJ, DAE and even previews Alembic (which is nice - save a trip to Houdini's command line abc viewer). It uses standard Maya navigation and previewing new geometry is simply a matter of drag/droping it into the viewport.

MeshLab is always handy to have for far more than 3d file viewing, but I'd also advise Open3dMod too. It's nowhere near as slick as FBX Review but it previews a greater number of file formats including LWO (it spits a lot of LWO's back out but it's one of the only LWO previewers I've come across (save for the command line tool in Houdini).

Apologies - I don't have time to provide the links but they're easily found on Google.

Thanks everyone for the recommendations.
Indeed the soft I used to use was meshlab, I will check out Open3dMod. Basically I have some heavy scans I would have liked to just preview in a quick way. Meshlab may do the trick.