Here is a small collection of applications I recommend and find either very cool or useful for your HTC Touch HD or any other Windows Mobile phone. These applications will unlock hidden powers inside your phone and make it even more user friendly better looking. So here’s my list, in no particular order…

FingerKeyboard 2
This great program replaces the onscreen keyboard on your touchscreen phone with one that is very handy and good looking. You can chose keyboard colors, create shortcuts and hotkeys for often used. For example you can by only one click write a smiley. Very useful, and free.
http://forum.xda-developers.com/showthread.php?t=501122 (added 20090722)

Auto Call Recorder
This application allows you to record both incoming and outgoing calls and save them, either automatically or by asking every call.
http://forum.xda-developers.com/showthread.php?t=536472 (added 20090722)

Change Screen
This application will rotate the screen automatically and you can configure it so exempt any program you like from certain rotations that perhaps does not support landscape mode. The program is launched at startup and is hidden from task manager.
http://forum.xda-developers.com/showthread.php?t=468488 (added 20090722)

mToolbox
This program has an angle ruler, water level, light sensor and shock sensor that might come in handy sometimes. It seems abit buggy, especially the ruler when calibrating, but I think it’s worth mentioning anyway.
http://forum.xda-developers.com/showthread.php?t=535944 (added 20090722)

15 tabs
This modification of the TouchFLO3D interface adds tabs like call history and calendar. It comes in many different languages but there might be some small issues with it, for example one cannot change the order of the tabs it seems.
http://forum.xda-developers.com/showthread.php?t=450278 (added 20090722)

Use HTC headset to play/pause/next song
Install this program to allow yourself to play, pause and move on to the next song in the TouchFLO3D music player by using the button on the headset.
http://forum.xda-developers.com/showthread.php?t=502881 (added 20090722)

Advanced Configuration Tool
With this program you can modify alot of hidden settings in Windows Mobile that will come in very handy. This requires .NET 3.5 for Windows Mobile.
http://forum.xda-developers.com/showthread.php?t=418151 (added 20090722)

CorePlayer Mobile
This is probably the best media player for movies out there right now for Windows Mobile. If you watch alot of movies with your phone this application is probably worth it.
http://coreplayer.com/content/view/28/69/ (added 20090722)

Diamond Light Saber
The famous light saber app for Windows Mobile.
http://www.freewarepocketpc.net/ppc-download-diamond-light-saber.html (added 20090722)

Garmin Mobile XT
This is a very good GPS navigator software that works well with Windows Mobile and HTC Touch HD. Another alternative is to you the built in Google Maps if you have a flatrate deal for data traffic with your mobile.
https://buy.garmin.com/shop/shop.do?cID=138&pID=11413 (added 20090722)

StopWatch
A pretty good and nice looking stop watch. The only problem is that if the phone goes into sleep mode (screen goes black), the timer will stop or significantly slow down. So if you want to time something the screen must be on, that’s not good.
http://forum.xda-developers.com/showthread.php?t=421812 (added 20090722)

Nimbuzz
In my opinion the best and best looking instant messaging software for your mobile phone. It can handle MSN, ICQ, Facebook, Twitter and much more. Very nice interface. For my HTC Touch HD I used the CAB-file for HTC Touch Pro, worked just fine.
http://www.nimbuzz.com (added 20090724)

More apps will be added as I find them. Feel free to enter your own suggestions in the comments.