Two of my favorite pieces are Rothbard's In Defense of "Extreme Apriorism" and Hoppe's In Defense of Extreme Rationalism. Also of great interest to me is the idea of extending praxeology--e.g., as Hoppe does in his argumentation ethics--and The Other Fields of Praxeology: War, Games, Voting... and Ethics?. This post mentions Adam Knott's interesting working paper, Rothbardian-Randian Ethics and The Coming Methodenstreit in Libertarian Ethical Science; Knott has various praxeology sites too.
A couple of older works I've been wanting to get around to for some time are John M. McTaggart's Nature of Existence and other works, and Leonard Nelson's System of Ethics. [Update: for more on McTaggart, see the Wikipedia entry; John McTaggart’s The Nature of Existence; Stanford Encyclopedia of Philosophy entry on McTaggart; ] David Gordon long ago mentioned McTaggart to me, who apparently tried to deduce the nature of the universe from the proposition "something exists"; he also argued that it can't be false that something exists. Gordon also told me Nelson has a very interesting Kantian-style ethic which relies on self-refutation arguments. Of course these are not praxeology but given Hoppe's argumentation ethics extension of praxeology these seem to be very interesting.
Update: Praxeology and Ethics: Three Philosophers Considered--Rothbard, Hoppe, Searle(In this regard Per Christian Malloch is worth noting, e.g. his interesting Amoralism in One Lesson, a defense of a hypothetical approach to ethics.)
There's also Conrad Schneiker's The Supreme Scientific Method Of (Universal Value Logic) Praxeology which, he notes, "is the Universally-Supreme Logical-Value System of (Axiomatic, Reflexive, Praxeological, Intensional) Logic, Which is the Provably-Universal Ethical-Logic of All Genuinely Realistic Thinking and is "the Logical Telescope of the Second Scientific Revolution". Ahem. And, shades of Mises, Schneiker's The Provably Ultimate Foundation of Science. And of cousre, there are others, such as the Axiomatic Theory of Economics.
Some others worth noting here are Steven Yates's De-Kanting Mises and Hoppe: Notes Toward an Austrian-School Metaphysics; Jude Chua Soo Meng's Hopp(e)ing Onto New Ground: A Rothbardian Proposal for Thomistic Natural Law as the Basis for Hans-Hermann Hoppe's Praxeological Defense of Private Property; and Barry Smith's In Defense of Extreme (Fallibilistic) Apriorism. Food for thought.