The jqModal is an "oldie but goodie" plugin. It's been around for a while and revved a few times and has been pretty reliable for what we've used it for.

http://dev.iceburg.net/jquery/jqModal/

I've gotta admit, initially getting it working was a bit frustrating for me at first. At the time (a while back) I had just started exploring jQuery modal plugins and wasn't really sure which would do the job. I sunk some time into this and after some initial stumbling, gave up. Then I messed with some others and was like, "fark, that sucked" (especially with cross-browser checks), and came back to this one.

Glad I did. Once I figured out how to get it set up (the css matters a lot, take a good look at the examples), it did the job and I've used it in a bunch of projects since then, which also gave me some opportunities to try more of it's features. All panned out well.

This is a nice one to use when something like jQuery Thickbox feels like overkill and you're really looking mainly to show dialogs and messages. We've had some other cases where we needed to show external sites in the modal and for those cases opted for Thickbox or the jQuery UI dialog widget, though there's a 3rd party example of opening an external site at the bottom of the jqModal examples (I haven't tried it).

jQuery pop is a simple little plugin that lets you put an up/down arrow anywhere on your page and when clicked it shows some content in a dropdown div. Handy for those times when you need a mini "menu" effect for a single item, but don't want all the overhead of a menu plugin (and don't need fly-out submenu's).

http://pop.seaofclouds.com/

I haven't actually used this one yet, but got it queued up for a little spin.

The jQuery emptyonclick plugin solves a common problem nicely, creating "watermark" text inside a form field.

http://www.madewithlove.be/blog/the-jquery-emptyonclick-plugin/

I've actually tried a few plugins for this kind of effect and most solutions have some kind of problem with them. There are a few subtleties to solving the problem nicely. I ran some tests on this one and didn't find any gotcha's or special requirements that would make it seem annoying to use.

Worth remembering. Solves a seemingly simple problem in a simple seeming way.

A note: if the form that the field you're watermarking has reset or submit buttons, an event handler gets bound to them that will clear the value in your watermarked fields if it's the same value as the default for the field. That will probably be useful and probably won't get in your way, but it's something to be aware of.

OK, gotta admit, this one's got me a little excited, probably because I've been playing with iPhone apps so much lately. On the iPhone (and some dashboard widget apps) there's a cool effect where you can "turn over" something you're looking at, which shows you what looks like the "other side" of it. A good example is looking at an image and then flipping it over to see picture info (date taken, etc). I've seen this effect used very well on a few web sites lately, too. Anyway, the jquery Flip! plugin:

http://lab.smashup.it/flip/

I've tested this in IE6 and it worked, FWIW. The author says it would, but I'm a bit paranoid (kicked enough times by that piece of merde). I'd like to see some more clear examples there of using the "content" option, which to me is what makes the use case something more than just a neat effect.

Anyway, this is kinda cool, IMO. Maybe "flip will be the next reflection", which was um, "the next rounded corners", which was um... the next Windows Playskool UI, no, wait, no... yes.

If you need to fade a set of images in and out of each other, all in the same box, this jQuery slide show animation plugin might do the trick for you:

http://www.gruppo4.com/~tobia/cross-slide.shtml

This can give you some of the nice fades and transitions that used to be found mostly in Flash-based solutions. The example there are nice, but you also can have a lot of control over how the animations look (think Ken Burns effect, with panning, zooming and fading all going on at once).

I fired this thing up in IE6 to see if it would pass muster and the examples worked. Kudo's to the author (Tobia Conforto) for that. This is a nice plugin to remember when you want to put in some nice self-contained image gallery animations.

If you're in the market for something for this, you might also want to take a look at the jQuery Cycle Plugin, which I've noticed is well supported in the jquery Google group (and I've used it for a few things, with success).