Searching for molecular structure formula software

[Ichneumon]

So... as I'm following a class BioChemistry, I need to draw lots of structural formulas of all kinds of molecules and I was wondering what type of program people use to write them on the computer, I've seen pictures like these:

http://upload.wikimedia.org/wikipedia/commons/thumb/3/3a/L-selenocysteine-2D-skeletal.png/180px-L-selenocysteine-2D-skeletal.pnghttp://upload.wikimedia.org/wikipedia/commons/thumb/6/6f/Ethanol_flat_structure.png/100px-Ethanol_flat_structure.pnghttp://upload.wikimedia.org/wikipedia/commons/thumb/c/cf/Adenine_chemical_structure.png/150px-Adenine_chemical_structure.png

And I just know there must be software to do these things nice and clean. I don't know where to look so I turn towards my fellow playgrounders and ask: Do you know what sofware people use for these things? And where can you get it?

EDIT: Also, I'm just assuming this software is free and open source.

Thanks

[Mordokai]

Tried ChemSketch (http://www.acdlabs.com/download/)?

[Ichneumon]

Tried ChemSketch (http://www.acdlabs.com/download/)?

Tied it now, and it works quite well. Thanks!

[Mordokai]

Another option you have is RasMol (http://www.umass.edu/microbio/rasmol/), though this one serves more for drawing biological structures, like proteins and DNA. Still, look it up, you might find it interesting and/or useful.