Once you have used Latex for several times, Wiki-book on Latex "Latex" is the second stop for you. I also use it as a quick reference.
sciposter can be used produce excellent poster. One feature I like most is that it automatically balances the materials on each columns such that each column is almost of equal length.
beamer can be used produce professional slides, notes or handout. As long as your slides are formed in a logical way, i.e. with section, subsections and etc, you can have various types of navigation bars, change the fonts or layouts by just replacing sevral parameters.
listings can be used to include source code in you document, which can be syntaxily highlighted.
xfig can be used to draw diagrams which are inputed to latex later. To be successful, two things need to be done on xfig sides: set special symbol to ON and font to Latex font. When you finish the drawing, save the file and export it as combined pdf. In latex, use \input to include the figure.