Depends on your definition of cool...
I wrote a small little weather utility application, which parsed METAR weather data given an city ICAO code. It was a Windows system tray program, similar to others that exist today, but less featureful. It would update the weather every X number of minutes, dynamically redrawing the icon as a thermometer to indicate tempeture. Blue was cold and red was hot. When you maximized the window you had a detailed description of the weather such barometric pressure, etc, as well as wind, direction and what have you.
I wrote that program years ago, using only SDK C style windows API calls. I read a flying magazine which had a link to real time METAR reports on the NOAA web site, did some research and cranked out that little app.
It was sorta ground breaking in those days, although I never released publically...I used showed the guys at the local RC flying club I belonged to at the time and we used it for our flights.
I also wrote an Windows system tray utility which acted as a proxy server capturing HTTP requests, HTML, etc and modifying it dynamically before display in IE. It used SQLite as a database to store web site passwords.
It would scan HTML and search for FORM elements and compare those elements to the database of previously entered passwords, if the URL matched as well as the form element names, the proxy would send the login credentials for you and return the page as an authenticated user. I hate remembering passwords.
If the credentials didn't exist and you sumbitted them a little popup similar to ZoneAlarm would ask if you desired to store those passwords for a later date.
Atleast 3-4 years ago I wrote that and AFAIK it was ground breaking as I still don't know of anything that does quite that just yet. I wanted to add auto password change functionality to it, but as CAPTCHA started gaining steam I realized that little app had seen it's day and decided to scrap it. To bad cause it had alot of potential I think.
I hacked a default Windows EDIT control using some undocumented techniques which I learned over time to display hyperlinks for URL's, etc.
I later investigated the possibility of turning it into a full blown colorizing code editor...which I decided was impractical although possible. That control I deemed worthy of an article on codeproject.com
http://www.codeproject.com/editctrl/multi_hyperedit.asp
The coolest application I have ever developed was an entire system of sub-systems.
Done in PHP/MySQL in 2002 sometime. Admittedly it was horrible code in retrospect, but the mindset of enterprise software developers and hackers are quite different. It worked and it did what he wanted, but wasn't very maintainable. I estimate around 7,000 lines of adhoc PHP
It included:
- Knowledgebase
- CRM
- Newsletter
- Resource management (files, etc)
- Interactive classroom intergrated with the KB
- 3 tier user role management
- Calendars
And a few more things...it started by replacing open source programs which he found and were extremely buggy...my first introduction into using open source PHP applications...was seriously disappointing
a Cross browser DOM library which I'm working on...as well as my own PHP framework mimicking that of Zend I also consider interesting.
Likely most complex project would be a template engine I have been working on in theory only, which doesn't require any logic but is fully capable of rendering all but
maybe the most complicated views.
My CMS:
http://www.sourceforge.net/projects/texocms
Although not ground breaking, it takes a different approach than most in how it creates and manages "web pages"
I have done tons of things I personally considered cool...some very cool...very innovative...I experiment alot...in fact only in the last 3-4 years have I considered following/learning best practices and begin to develop more enterprise style applications (maintainability, proper documentation, etc).
You can't make money in experimenting in esoteric areas such as hacking a Windows control or tinkering with a Kernel. Although that is what I love to do...it's difficult to make money doing it.
PHP has made it light years easier to develope large enterprise applications which I can totally appreciate. As cool as it is to tinker at a low level or desicover undocumented caveats, etc...PHP will actually make you money...which is always good.
Go PHP Go!!!
Go PHP Go!!!
Go PHP Go!!!
It's difficult to define cool applications or coolest code you've ever written...as everyones definition is different I guess. By the above are my opinions and experiences...
Cheers
