Rational Triangles
Triangles in the Euclidean plane such that all three sides are rational. With tables of Heronian and Pythagorean triples.
http://grail.cba.csuohio.edu/~somos/rattri.html

The Nautilus Spiral - A Sequence of Triangles
Studying the expanding sequence of isosceles right triangles utilizing recursive and explicit functions. With diagrams, explanations, and tables.
http://www.theoldpro.net/ms/nautilusproblem

Polyiamond Exclusion
Colonel Sicherman asks what fraction of the triangles need to be removed from a regular triangular tiling of the plane, in order to make sure that the remaining triangles contain no copy of a given polyiamond.
http://www.monmouth.com/~colonel/xpoly/xpoly.html

The Golden Pentagon
MathPages depiction of tiling a plane using a progression of equilateral triangles dividing a pentagon. Most interesting is that this pentagon is uniquely and recursively divided by triangles, just as is seen in the more common recursive division of the Golden Rectangle.
http://mathpages.com/home/kmath153.htm

Elliptic Curves and Right Triangles
Slides (GIF) of lectures by Karl Rubin at Stanford University.
http://math.stanford.edu/~rubin/lectures/sumo/

Pythagorean Triples Etcetera
Pythagorean Triples, etc.
http://www.geocities.com/fredlb37/

Schröder Triangles, Paths, and Parallelogram Polyominoes
Schröder Triangles, Paths, and Parallelogram Polyominoes
http://diamond.boisestate.edu/~sulanke/PAPER1/PergolaSulanke/PergolaSulanke.html

Hexiamonds
The Social Media blog is your single destination for updates on the AIM Buddy blog, the AIM Pages blog, the AOL Community Info blog, the Expressions Factory, the Marvelous Mob and the Social Media Mess blog.
http://members.aol.com/huttlin/hexiamonds.html

Tim's Triangular Page
Explains the basic properties, formulas, theorems and different types of triangles. Includes proofs and illustrations.
http://sakharov.net/triangle.html

Rite Item
Rite Item presents its AnyAngle triangle solution program description and download sites and a link to Jim Preston's personal home page
http://www.riteitem.com/