jQuery Tools has become a disaster... I think Tero got in way over his head trying to rival jQuery UI and just no time to support the product. I tried it for a while and his methods are certainly unique. He turned the jQuery tag-line of "write less, do more" completely upside down and in my opinion, only does himself and the jQuery community a disservice. He rarely posts in his own forums and has not released an update in over six months despite more and more incompatibilities with the ever updating jQuery.
I really like prettyPhoto for the Overlay Lightbox effect. It's well supported and dynamically re-sizes itself to properly & automatically fit any content. It supports titles and captions as well as an optional mini thumbnail gallery inside of the lightbox itself.
There are also six skins (themes) included which are based on popular site designs.
There's nothing wrong with Fancybox at all, but prettyPhoto has a lot more cool features enabled right out of the box. Literally only takes a few minutes to set it up to function like the demos.
It also plays nice with other plugins. I use jCarousel to display my thumbnails and prettyPhoto to zoom in on them when clicked.
jCarousel takes a bit more CSS fiddling to look they way you want but both of these plugins are fully customizable. If you know your CSS, you should have no trouble duplicating the exact look of your old jQuery Tools applications.
EDIT:
I agree with others on the following...
qTip2 is an excellent ToolTips plugin and I think the jQuery Validation Plugin is really the only way to do jQuery form validation.
EDIT 2:
FancyBox 2 is great and more solid than prettyPhoto in my opinion. There are animated opening and closing effects which you don't get with prettyPhoto. If you don't need the mini thumbnail slideshow within the lightbox, I prefer FancyBox 2 over prettyPhoto.
jCarousel has a major bug in circular wrap that the developer has acknowledged yet fails to fix. You cannot wrap around. Even the online demos are broken. jCarousel Lite does not have any issues at all with circular wrapping.
Excellent, VERY popular and solid, non-carousel style slide-show plugins with transition effects include Nivo Slider and Cycle.
EDIT 3:
I've recently discovered Tooltipster and now heavily favor it over qTip2.
