Lately i'm having a lot of fun with TrueSculpt, an android app that literally gives us the power to create 3D meshes in the tip of one's finger. I'ts a simple app that allows mudbox style modifications to a primitive sphere. Exports as an OBJ file, making it useful to create content for other 3D applications. The final results have an high polygon count, making it not very suitable for VRML worlds where size and bandwidth are at a premium. Still, it's a fun app, unique as far as i know in the android environment (iOS has a lot of 3D modeling apps...) and an harbinger of things to come. Find it as a free download in the google play app market and check the google code project here: Truesculpt.
That is a nice android app.
A free similar program for windows and Mac is Sculptris
http://www.pixologic.com/sculptris/
It's a reduced version of the well known zbrush but also easy to use and powerfull. Has a feature to reduce polygon count and also exports to obj format.
vcard
Hmmm... knew about it, but sadly haven't tried it extensively. For this sort of modeling i generally go with CB Model, but since Dassault Systèmes decided to focus on 3DVia development stopped and the program is getting a bit long in the tooth...