X-Lite - SIP client. I tried some open source soft phones and couldn't find one that actually worked. X-Lite sure looks silly, but it works. I use it with WorldDialPoint for those rare times I want to call people and not use my mobile phone cap. Unfortunately it doesn't seem to be supported on Snow Leopard. In fact I no longer like it at all - it doesn't work with WorldDialPoint on Windows or Mac OS X. I found that 3CX is good for Windows and the iPhone, but haven't found something I like for the Mac.
Isolator - thing that obscures everything other than the foreground application. Good if you get distracted easily like I do!
Mozy - 2GB of free online backup. Referral link means that you get more than 2GB. It stopped working mysteriously and then I changed to a place with a broadband connection that counted uploads so I've stopped using it for now. I now use Dropbox instead, which has come in very handy when I've forgotten to carry a copy of an important file and can log in to the iPhone app and email a link to myself.
firefox-mac-pdf - a Firefox plugin that lets you read PDFs in the browser window, which is the one thing I really liked about Safari. Since Safari 4 was released and in conjunction with SafariBlock, I can no longer think of a good reason to keep using Firefox, though.
Since Snow Leopard was released, SafariBlock no longer works. I installed GlimmerBlocker which is an HTTP proxy, but gives me all sorts of wonderful on-the-fly transformation options.
Parallels Desktop - I tried both Parallels Desktop 6 and VMWare Fusion 3.1. The former works very well with the exception of my PICkit 2 programmer, which works fine on my iMac (though the menu claims that the device isn't connected to the VM) but not at all on my laptop. It works perfectly in VMWare though. Both work at their peak when you have plenty of RAM - 8GB or more!
Occasionally standard keyboard shortcuts such as Alt-Tab stop working and the Dock doesn't display (when set to auto-hide). I found that killing the Dock process fixes this, at least until next time. I don't think this has happened to me for a long time - perhaps it was fixed in Snow Leopard?