pdfTeX is an extension of TeX which can produce PDF directly from TeX source, as well as TeX's original DVI output. pdfTeX incorporates the e-TeX extensions, and adds to them.
Perhaps the most notable added capability is microtypography line breaking features. The microtype package provides a convenient interface for LaTeX. pdfTeX also has extensions related to graphics, macro programming, system queries, and more.
pdfTeX is released as part of TeX Live, and also included in all TeX distributions, notably MiKTeX. Current release: NEWS, manual, sources.
The principal author, and still active maintainer, of pdfTeX is Hàn Thế Thành.
Upstream pdfTeX is a Subversion source repository at tug.org. For historical reasons, current development happens in the branches/stable area; perhaps this will change to the usual trunk/ in the future. Example for running svn from the command line:
svn list svn://tug.org/pdftex/branches/stable/
The TeX Live source repository also contains pdfTeX; libraries, infrastructure, and other related updates are generally made there. The TL pdftexdir source directory itself may or may not be in sync with the upstream sources at any given time.