Python

Python is an easy to learn but quite powerful programming language.
You can get a copy from http://www.python.org/
There are versions for Windows, Mac, and Unix/Linux.
Versions 2.x and 3.x have differences, see http://wiki.python.org/moin/Python2orPython3

Rosalind, http://rosalind.info, wants you to use the 2.7.5 version. 
The Coursera, https://www.coursera.org/, class "Computational Molecular Evolution", wants you to use 2.7.5.

The Windows versions come with the editor IDLE.  While you can load both 2.x and 3.x versions you will have trouble getting windows to allow you to associate the same file extension, py, in a way that will allow you to edit with either version by right clicking on the file.  I ended up installing 2.x first then 3.x later.  When I want to use 2.x version of IDLE I open a 2.x python shell and select file from the menu and select the file I want to work on.  The unfortunate side-effect is the working directory isn't automatically set.  I think I need to learn how to have this happen automatically.  It shouldn't be that hard.

So far I've only run into minor differences between 2.7.5 and 3.3.2.  I think I'll make my code work with either version if I can.

Python has several forms of collections.  So far I've used sets, lists, strings, tuples, and dictionaries.  Sets are unordered values where no value occurs more than once.  Lists are ordered.  The same values may appear multiple times in the same list collection.  strings are ordered sets of characters.  Tuples are an ordered set of unlike members.  Dictionaries are sets of key/value pairs.