Here are few apps I use on my daily basis and they are pretty handy
Audacity
Audacity is a audio editing tool. It is a really simple tool for quick audio editing. Audacity is available for Windows, Linux and Mac OS
Cyber Duck
Cyber Duck is a FTP tool. A handy tool for web developers, connect with their remote hosts.
Leech
Leech is a download manager which is really handy, simple and downloads your files pretty fast, but it is not free. However you can get a Trial Version from Leech Website. For me it turn out to be a great application, so I have purchased the application.
HandBrake
An awesome application to convert videos according to your need. It has preset profiles for your iPod, iPad, iPhone, universal, apple tv, etc..
Name Changer
Ever had a hard time renaming huge number of files? Well, this is a handy tool that could help you out. It is a simple tool only purposed for renaming files. Drag your files and set your configuration and click rename. Your done.
I believe function is available in Automator (the built in application in Mac OS X). But comparing both applications, for me Name Changer is much hazle free than Automator.