Here are a few applications that I use, why I use them, and where you can get them.
Things- A bit pricy, but the absolute best to-do and task manager out there. Perfect for managing different areas of you life- work, home, school, etc. Easy to use and looks beautiful. If you want a free alternative that is much smaller with simple to do lists, then
Anxiety is a good option.
Evernote- A great note taking and screen clipping app that syncs with their servers. Free or paid for more storage. It's great for taking notes in class or meeting; can even find some words in graphics.
Bowtie- Awesome little app that sits on your desktop and displays your itunes song, cover art, etc. Very skinable and flexible. You can hide this in your dock and it only appear when itunes is started.
Adium- Perfect multi-platform IM client. iChat messes up for me often.
iBank- Very useful application that can, in most cases, connect to your bank account and manage your finances, stocks, and multitude of other finance-related functions. I like this app a lot.
AppCleaner- Easiest way to delete applications and the traces that they would leave behind if you just dragged them to the trash.
AppFresh- Cool application to update all of your other, third-party applications. Very useful, a must need IMHO.
Caffeine- The most useful application for everyday use. Clicking the little coffee cup in the menu bar temporarily disables your mac from going to sleep or your screen dimming. Perfect for when you watch a video that lasts longer than your sleep/screen dimmer timer.
WeatherBug Lite- Display temperature in your menubar. Awesome, easy and lightweight. Click to access forecasts.
Little Snitch- Just because your computer is a Mac doesn't mean it doesn't need a firewall. Even the firewall in Leopard is not that great, nor enabled at the start. Little Snitch is a good, lightweight option.
ONYX- The one and only optimization/clean up app you'll ever need for Mac.
There are a few others, but instead of writing them all down, just check out the list I made a while back. I need to update it with a few apps from above, but it's still a very good list-
http://blog.jeremybuff.com/blog/best-mac-os-x-leopard-software/
I hope my input helps you out!