Calculation of the lowest energy conformation with a webbased solution or freeware
Avogadro can do a lot of this for you. It might not be precisely what you want but the molecular mechanics from Open Babel integrated into it can do at least some of the tasks you mentioned. It's free and has a very intuitive and smooth user interface. A real pleasure. The downside is that it's poorly documented.

To play around with conformations, go to Extensions -> Molecular Mechanics. To view the generated conformers (from the Conformer Search option) go to View -> Properties -> Conformer Properties...