Ubuntu is a free linux distribution which is easy to use.
most important packages to get after a fresh OS install:
vim-full, gnome-do, imagemagick, texlive, texlive-latex-extra, texlive-math-extra, texlive-science, latex-beamer, openssh-server, gnuplot, feynmf, vlc, gparted, vim-latexsuite (see install guide), gnome-do, dropbox, gimp
optional:
conky (FAQ), envy (for nvidia graphics card), wicd (for network), xchat, amarok, abiword, gnumeric, keepassx, freemind, calibre, filezilla, octave, qtoctave, wine, nautilus-open-terminal
nice games: wesnoth, gnubg, openarena