pdfTeX is an extension of TeX which can produce PDF directly from TeX source, as well as original DVI files. pdfTeX incorporates the e-TeX extensions.
pdfTeX also has a variety of other extensions, perhaps most notably for microtypography line breaking features. The microtype package provides a convenient interface for LaTeX.
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.
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.