Below are some tools that I found useful for my Macbook.
1. LockTight – provides the functionality of locking your MacBook with hotkey combination similar to CTRL+ALT+DEL in Windows
2. Paintbrush – Lightweight paint tool
3. Growl – Notification System. Provides alerts. I used along with GrowlMail plugin for Mail application to generate alert for New emails.
4. Adium – Instant Messaging. Can add MSN, Yahoo, GTalk and other social network accounts.
5. More will come as I find one.
You can checkout latest Mac apps in http://www.macupdate.com/
Feel free to suggest ones that you found yourself useful.