PGF is a TeX macro package for generating graphics. It is platform- and format-independent and works together with the most important TeX backend drivers, including pdftex and dvips. It comes with a user-friedly syntax layer called TikZ.
(via this post on the PLT list)
PGK and TikZ are great tools by the author of the Beamer package, which I also use quite heavily. Absolutely awesome for those which publish their graphs via pdflatex and who can’t use PSTricks.
I also quite like dot2tex(i) which converts Graphviz graphs into TikZ/PSTricks code that can be easily embedded inside LaTeX.