Open source tools
Open-source has really revolutionized software development. Recently, I was setting up a new notebook and that is when I realized how many open source and free tools I use (some of these are not really open source, rather you don't need to pay a license for a simple base version):
SQLYog: This is a decent GUI Client for MySQL and somewhat better than phpAdmin in terms of UI. Allows easy creation of DB, tables and editing of table schemas. It is also easy to edit data inline and import/export data. Toad has created a new MySQL client, but I haven't tried it yet.
NetBeans: excellent Java IDE. Other languages like PHP and Ruby are also supported (or in the works). I particularly like the code completion and popup help/hints.
Skype: Allows easy conferencing over the Internet. Makes it easy to stay in touch with other developers and talk using the headset and hence an ideal tool for entrepreneurs :). A webcam can also be integrated into this app.
Putty: to telnet and SSH into remote servers.
PDCreator: Allows easy creation of PDF documents. Don't want to send your resume in Word? Just output it to the PDFCreator to get an exact looking PDF doc.
Picasa: In addition to organizing your (large collection) of pics, it also allows easy creation of web albums. It then becomes very easy to publish the albums onto any web site.
Vyew.com: For online colleberation and desktop sharing. This one requires no installs and it becomes easy to show your desktop to others over the internet. Can be slow sometimes, though. Crossloop is another excellent app.
IrfanView: A Windows based graphics viewer supporting a large number of file formats. It is very easy to navigate a folder of images with this (just keep pressing the space bar). Also allows basic image editing.
Pidgin: provides a single interface to various IM clients like Yahoo Messenger, Google Talk, MSN, etc.
VLC Media player: plays almost all Video formats including flv files.
iTunes: Makes it easy to manage mp3 songs. Even though I buy very few songs online, it allows me to easily manage songs, burn mp3/regular CDs and edit the meta tag on mp3 songs.
SmartFTP Client: FTP client with an easy interface
Real Alternative: Allows you to play real media files
RealVNC: allows me to connect to a remote computer and control it. No, I am not hacking anyone.
Firefox w add-ons: Does not need a reason. The add-ons are so many, you have to actually pick and choose. The favorite ones include web developer, download manager, firebug, delicious, weather, google notebook.
del.icio.us: provides a convenient storage for online bookmarks. The firefox plugin makes additions and access easier.
Fiddler: allows debugging of HTTP traffic between computer and Internet. Want to 'see' SSL traffic? It decrypts it for you.