Well, I ended up mucking mine up a few times requiring a hard reset or 3 to fix the glitches. Some of the software you're going to find and try won't always restore the origional settings when it's uninstalled. Or, it may not be immediately apparent which app is giving you a problem. So I recommend installing the trial for either Sprite Backup or SPB backup (You'll find fans of both here) and making regular backups of your device as you install and try new software. Then you have something to revert to if there're problems.
First site I found for apps was
http://www.aximsite.com/boards/appli...ing-point.html
It has a lot of freeware/shareware. Stay away from NoteM, that was my first personal nightmare... It breaks the file associations with WMP. They can be fixed with registry editors, but that takes research I hadn't done yet.
I like Resco registry editor for edits on the device, and Mobile Registry Editor for edits via activesync. I also recommend My Mobiler for RC of your handheld. (sometimes it's just easier to use a full keyboard.)
Read reviews before you install, it can save a lot of headaches. And be aware, that while a lot of today apps can be flashy, they can also suck up a lot of prescious memory. (SPB Mobile Shell, SPB Pocket PC and Resco Today plugin immediately come to mind as memory hogs on the today screen.)
Additionally, check reviews and/or install notes to see if an app can be installed on the storage card, or needs to be on the device. And note that even if an app can be installed on the card, if it needs fast access; a today app, ringer, calander, event-triggered, it will be slower on the card.
Go slow, make sure you have a recent backup, and have fun with it. This is a great site for help and questions, just remember the cardinal rule and *search first*.