Optimized Firefox
In my opinion, Firefox is the one internet browser that now computer should be without. And now with unofficial builds optimized for Intel and G5 Macs you have no excuse. The one thing that keeps me with Firefox is its extreme customization with extensions and themes.
Vienna
Vienna is a free open source rss reader for OS X. It features a built in tabbed browser, custom article display styles, smart folders, full Apple Script support, and many other features. I use Vienna everyday, and I love this app.
Adium
Adium is a free instant messaging application. It is extremly customizable, and can connect to a variety of IMing services including the popular AIM, MSN, Jabber, Yahoo, and many more. I like to think of Adium as the Firefox of instant messangers as far as custimizability goes.
Chax
If you aren't using Adium, you definitely need to be using Chax with iChat. Chax is a plugin for iChat that offers many additional features such as tabbed chatting. With Chax you can also set iChat to auto accept text chats. iChat is not complete without Chax.
NetFixer
NetFixer is a tiny little app that allows you to take screen grabs of whole websites. All you have to do is put the url into NetFixer and click Shoot! The image can be saved in as a png, bmp, jpg, gif, or tiff, and you can also define the size of your canvas.
BoxCloud
File sharing at it's simplest. BoxCloud's "drag and share" functionality makes sharing files a snap through a small application you install on your desktop. You decide what to share simply by dragging files from your desktop over your buddy list. Your files and folders are hosted directly and instantly from your computer without any uploading. Once shared, your contacts can then access your files over a regular browser -- they don't need to download or configure anything. If you want, you BoxCloud's servers can even cache the file for a week, so if your computer is offline, your file can still be downloaded.
SunriseBrowser
SunriseBrowser is the light and fast open-source web browser for developers. It features auto resizeable windows, transparency, graphical bookmarks, and many other features, such as a page shot tool to take a screen shot of the whole webpage or just the part being viewed.
iColors
iColors is a simple little app that gives you the hexadecimal code for any color. It's great for web developers, and better than an HTML color chart that usually doesn't have the full range of colors. iColors will even give you the hex for any color on your monitor, and you can also save those colors and come back to the later.
Taco HTML Edit
Taco HTML Edit is a full on HTML and PHP editor for OS X. Taco HTML Edit has many features including a built in spell checker, live previewing, and syntax checking. It's light-wieght and best of all, it's free! I've used Taco HTML Edit a lot, that's even what I created my email signature in. It's a great little HTML editor.