jGrowl is a jQuery plugin that raises unobtrusive messages within the browser, similar to the way that OS X's Growl Framework works:

http://www.stanlemon.net/projects/jgrowl.html

Messages appear in a corner of the screen, then fade out. Can display 'strips' (for lack of a word) of sequential messages. Some people call this a toaster effect: like toast coming out of the toaster (and then more toast out of the same hole (hmm)).

[2 cents] Lots of options here...well explained, a good set of examples (but none for putting messages in the viewport bottom-right corner, which is where I'd probably use it). 7 releases so far, so it seems pretty active (though no dates listed and I didn't poke around). Definitely on my keeper list when the muse-case strikes.