This page gives an outline of Mac OS X projects I've worked on and written about. Note - most of the smaller hacks are not guaranteed to run on new systems - I haven't tested many of them on 10.4+ or Intel macs. Please do let me know if you try to use something here and find a bug.
Full Apps
Webmail.app
A distraction-free browser for GMail. See the original blog post here: "Webmail is not web browsing". A very simple use of WebKit, the contribution here is knowing what to take away. This was an idea that caught on all over, and has spawned a number of site-specific browsers, and even site-specific browser generators. See webkit.pbwiki.com for relevant links.
BibDesk
The first and largest of my projects, BibDesk is a Mac OS X BibTeX bibliography manager with a mix of powerful features for searching online libraries and organizing references with local copies of papers, then quickly finding the right reference and inserting it without interrupting writing.
Blapp
Mothballed since 2006, this was my claim to fame in the blogging revolution. Blapp is a simple Blosxom weblog editor - it only has the frills you need. Efficient instant preview, auto-save, interoperability with NetNewsWire and a links window to search your past posts made Blapp a great tool for Blosxom weblog users.
Both Blapp and BibDesk have had significant contributions from other generous hackers - check their docs for acknowledgements. Those programs certainly wouldn't be worth much if it was just me using them.
Smaller Projects and Demonstration Hacks:
Stakeout
A project to support running automated tests when files change, by watching for file changes at the system level using kqueue. Source.
Incremental-Search InputManager
Incremental Search Plugin for NSTextView
An InputManager that adds emacs-style Incremental Search to any app using NSTextView.
Screenshot:
I-Search Plugin - download binary and source
bindlist.py
Possibly outdated after the release of OS X 10.5, this utility parsed the details of nibtool's output to give a listing of bindings in a nib and what they're connected to, in an attempt to remove some of the invisibility problem that can make bindings hard to debug. Source
fcd
A small program that I wrote to demonstrate integrating the Mac OS X Panther Finder's Sidebar into the command line shell. Get the binary or the source.
WebKitPolicyExample
WebKitPolicyExample is a program I wrote to show one way to intercept clicks into a WebView so you can use generated HTML as a user interface element.
WebKitPolicyExample - download source
SelfService
An application that shows a floating window for selected system services. You can use the services by drag n' drop. System services are really useful, but the existing ways of getting at them are awful - adding to the crowd of key shortcuts or relying on a huge unwieldy hierarchical menu.
Screenshot: 
SelfService - download (free)
AddressService
A system service bundle that adds two services that search the OS X Address Book Database. Just highlight someone's name in a text editor and choose Address/Insert Address or Address/Insert Phone Number to replace the selection with their name and adresses or numbers. It searches for the string as a substring in first or last names. It's really great for when you're writing an email and you want to send some info about someone, even yourself.
AddressService - download
TextShapeView
An exploration of another way to navigate text - by the shape of its lines, zoomed out to 1px per line.
Download
TextStructureView
Another exploration of how to navigate text - this time, by revealing its structure as an outline view. This was implemented as an NSTextView input manager hack, and had the ability to plug in different text-structure definitions, like latex, XML, or Objective-C. Here's a screenshot: 
XRA
XRA is the beginnings of a system for contextual info, aka "implicit query" like dashboard for OS X. The idea is that there's a standard plugin (An NSTextView inputmanager) that loads other plugins, and sends the text as a query to the other plugins. Those plugins can do whatever they want -- for now I only have one plugin, a wrapper for the Remembrance Agent. This is already pretty cool, but it needs some work. I posted about this on my blog
There isn't a release package anymore, but the source is still available in a GNU Arch archive here: xra--main--1.0.