All applications that support AppleScript will work, and you can tag any file from the finder by highlighting it and doing Control-Space bar, type in the tag for the object, and hit return. Multiple items can be selected for fast tagging, and you can create a list of frequently used tags to apply to projects. If you are tagging some emails, the procedure is the same. Highlight one or more messages, do Control-Space bar, add your tag and hit the enter key. Done. Option-Space bar lets you search.
It seems like a bit of a hassle to tag things, but in fact once you do it, it's fast and efficient. You can have multiple tags for any item, and multiple files can have the same tag. You don't have to know where things are on your Mac. But you can find all the related items in a second or two.
Tags has some limitations. iPhoto is supported, but you can't tag smart folders. Same with iTunes. Stickies aren't supported, and iCal isn't either.
MS Office works fine with Tags, as do the Apple Finder, Keynote, Pages, Numbers, Mail, Safari, Photoshop and many more applications. The Gravity Apps website has a lot more detail, and several videos that explain the process.
At first, the usefulness of tagging is elusive. It was to me anyway, even though I use it every day in other contexts, like tagging photos in Flickr, or using Gmail. Having tags almost everywhere on my Mac is a major change in my daily workflow, and it is a change for the better.
Tags requires Mac OS X 10.6 "Snow Leopard " and sells for US $29.00 for a single license. TUAW readers can buy the app with a 15% discount ($24.65) using the coupon code TUAW at checkout. The coupon is good until October 31. You can also try the program free for 15 days.
For some, having this level of control of your accumulated data will be life changing. I think Tags is worth serious consideration if you want better management of your data and want to stop wasting time looking for things you already have.