Says Rodrigo: These notifications expire a few seconds after they are shown, so they should not disturb the user at all.

The first thing that came to mind when I read this was that this line is the exact justification for popup advertisements in browsers. Some more advanced popups using DIV even expire themselves, so they meet pretty much the exact same criteria as notification popus.

I'm still surprised why so many people, especially those which do a lot of usability work in Gnome think it's a good idea to use popups. Popups do two things that have been identified as very bad in a lot of other contexts:

1. They occupy screen estate not assigned to them. To keep his screenshots as an example: The bottom right is where a lot of status bar icons in applications are.
2. They are animated. They're not containing animations, but the popping up and fading out of the notification is an animation. Unexpected animations cause interrupts in workflow. OTOH, Clippy is really cute, and you can switch him to look like Einstein.
3. Another thing not related to popups but related to libnotify is discoverability. If I'm away from my computer for 10 seconds I might have missed a notification.

So the question is how notifications should be handled? Well, there's different kinds of notifications:

1. Notifications the user wants to be interrupted for. Such notifications would be allowed and even should try to interrupt the user. Examples here are low battery on laptops, full hard drive, but also me waiting for a download to finish or a friend to come online and just browsing the web until that happens.
2. Notifications the user wants to look at later. This is mostly stuff where the user decided that things he's interested in haven't happened yet and then diverts his attention elsewhere, but will come back later to check. Examples here are downloading a movie someone linked to, checking email or friends going on/offline in IM.
3. Notifications that are uninteresting and the user will probably never want to look at. Examples here are all the join/part messages on IRC or IM, the temperature outside changing or the clock ticking.

I think these 3 categories pretty much sum up the different uses for notifications. At least I've not found any important steps missing. But feel free to enlighten me. An important thing to notice is that for different users different messages are important.

I think I'll now just sum up some ways that I have found for notification handling and how I feel about them.

• Windows XP pops up bubbles all over the place telling you things like "Where are my icons gone?" or "Where are my programs?", especially after installation to get people aware of the new features compared to older Windows versions. After I have installed XP for the fifth time, this is just annoying.
• Firefox (at least on Windows) pops up a little box in the top right to inform that all downloads are complete. Apart from the fact that it doesn't work correctly with fullscreen applications (read: games), it mostly fails to notify me of what I'm interested in: Most of the time it's the first download to complete, not the last. And there's the issue of discoverability. If I've been looking away for a short while, the box has already disappeared, so I might not notice the download having finished.
• All browsers (and also IRC and chat programs) use Icons or colors in the tabs to tell you when a tab has finished loading. This is a really nice notification, because it uses a space pre-assigned for this task and is easily discoverable. (Imagine libnotify would popup a little box for every page that finished loading... ;))
• Applications on Windows are allowed to flash their taskbar entry to notify. This is a nice feature, because it only occupies space assigned to them. Slightly bad things about this from my point of view is that it's constantly blinking, which tends to annoy me so I click the app to turn it off - I'd prefer if it'd just change the color of the taskbar and not blink and there's sometimes and issue with discoverability if I'm using an auto-hiding panel.
• Amarok uses an OSD for song change notification. While a funny little gimmick, it really gets annoying when something pops up on your screen every 3 minutes.
• Lots of applications have voice messages to signal notifications. Those are quite nice, unless I'm listening to something important (like when coding audio applications), don't want to be interrupted (like when coding), when I'm not at my desk (discoverability again) or when they happen far to often (Teamspeak's "player joined" message really is that annoying). In the end they're a great addition, but not a good mechanism in itself.
• Warcraft 3, Unreal Tournament and I suppose lots of other games too print notification texts pretty much centered and in a big font onto the screen. This works very well even though it's essentially a popup. The reasons for this I think are: 1) The message doesn't really occupy a lot of screen since the background is transparent and I only see the text message (slowly fading out), so I can easily see what's happening behind it. 2) The message is important, so I want to read it anyway. 3) There's no discoverability problem, because while playing I'm not away. 4) I don't perceive it as an animation while it's fading out, so I'll look at it when it pops up and can ignore it after that.
• Warcraft 3 offers a machanism to jump to the current notification. In Warcraft 3 this is a huge feature since it allows centering the view at where the action's at. Especially since the key is unused otherwise. The key is space btw.

So, summing it up, I think I like notifications that don't occupy screen estate they weren't supposed to use, don't divert my attention when they're not important to me and are easily discoverable even when I've not been there for a while. I have no clue how to ideally do that, but I know that the applet popups from Rodrigo fail on all 3 of these points.