Mr. Moniz here,

I thought I might mention some apps I've discovered since switching over to OS X that I've been thoroughly enjoying (I'll leave my migration rant for another time).

Most recently, breve is a neato-keen donationware Cocoa simulation lab for alife, genetic algorithms, and the like which makes ample and very nice use of OpenGL. It's fully hackable with it's built-in object-oriented simulation language, "steve" (no relation to Mr. Jenson), which takes most of it's cues from Objective C. There's also a plug-in API which you can download if you want to hack breve plug-ins in Objective C itself (with Apple's OS X Developer Tools).

And now, in no particular order...

F-Script is a neato-keen freeware open source Cocoa scripting environment for OS X that takes it's cues from Smalltalk and array-oriented languages such as APL. It allows you to dynamically design apps using Cocoa objects but through a Smalltalk/APL hybrid scripting language (although, truth be told, it's mostly Smalltalk-ish with bits of APL essence thrown in where it helps out, and since there's no real APL character set nastiness one could even say it's more like bits of J essence). Check out the new FScripter tool as well.

TeXShop is a Cocoa TeX/LaTeX editor and previewer that makes use of teTeX and the Quartz DisplayPDF layer to go directly to PDF.

Fire is a freeware open source Cocoa multiprotocol client for many instant messaging services including AIM, MSN Messenger, Yahoo! Messenger, ICQ, and Jabber.

And, finally, for no particular reason,

Graph-O-Matic is a shareware Cocoa 2D and 3D graphing calculator by the maker of breve with extensive OpenGL support.