I hear a few people really raving about Apple's Dashboard. Personally, I've found it only mildly useful. Most things take up system ram and resources that I don't really want to sacrifice.
When OS 10.4 first came out with Dashboard I (like everyone else) jumped on it and loaded up every widget we could get our hands on. I had been using Konfabulator (I even purchased a license) with a couple of widgets, but Dashboard looked so darn sexy that I piled every known applet onto my dashboard. It only took a few weeks before I was tired of the system slow down and the cluttered look of dashboard.
Slowly, I filtered through all of the widgets and settled on just a few that are truly useful. Of course I use Apple's Calendar, Clock and Stock ticker (I use Quicksilver for my calculator), but my favorite widgets are not provided by Apple. Here is my list of third party widgets that actually bring something useful to the user experience.
Weatherbug is everything that the Apple Weather widget is not. It is accurate and provides some useful extras. After you enter your zip code, you get to choose from several local weather choices. After selecting the one that is closest, you get up to the minute local weather conditions with an ACCURATE three day forecast. You can also directly view a selection of local live video cams and weather radar maps. To top it all off, it is free. Sure, it's not quite as pretty as Apple's widget, but it sure is more useful.
Note pad by Widget Machine is a great little widget to replace Apple's pathetic Stickies. I purchased it along with Stratosphere and VoiceNotes awhile back on MacZOT. It was a good deal, but the only one that is still in use on either of my Macs is Note pad. It does what you want your notes to do. Multiple pages, security, global search are just a few of the more useful features. I highly recommend this one. VoiceNotes was also a good little app on my Powerbook when I took a Physiology course last year. Just as the name implies, it allows you to capture voice notes instantly through the Dashboard. Alas, I found that my Ipod with a microphone by Griffin worked much better and was easier to keep on those miniscule desks in the lecture hall in the Harvard Science building.
Finally, there is App Update. This one should have been part of the OS. The widget simply compares all of the installed applications on your Mac with the versions available through Apple, Versiontracker and Mac Update and lists any differences. It has a beautiful interface, does a great job, and is donation ware. It will even shrink to a small icon to save Dashboard space. There's also a widget update version as well. But since I run so few widgets, and they all are working smoothly, I see no reason to update any of them right now.