InfoHub - The biggest index: 10,000+ tours, 100+ specialties, 170+ countries. Tends toward popular, light-adventure (cooking schools, art lessons, etc.)--unlike the others, it also lists plenty of traditional escorted tours. You can't link directly to an operator; you fill out a form that gets sent to the tour company, which contacts you (this roundabout protects infoHub's revenue stream of commission by proving you found the tour through them). Specialty Travel Index - You want to find the very best operators of bike tours to Ireland, cooking classes in Provence, art history tours in Tuscany, classical culture cruises of the Aegean? Log on to this secret weapon of travel agents (not to mention travel writers), an index of the 600 or so best specialty and adventure travel outfits on the Web. Direct links to operators. Shaw Guides - Similar to Specialty Travel Index, but much less vetted, so there's an embarrassment of choices available--more than 5,100 of them, from the biggest established tour operators to the single ex-pat living abroad who leads walking tours of town on his day off to earn some extra dough. Anyone can list themselves. Some travel outfits and tour operators are reviewed, some are merely listed; all are direct-linked. I've heard it's no longer updated, but the site's still working and a fab resource.
I know I've railed against guided or escorted tours on principal, but there is one sort of tour I'll abide, and that's the specialty tour. We're talking any sort of themed vacation where you learn something, travel in some special manner (by kayak; with a bunch of professors), or otherwise get a shortcut to getting actively involved with some aspect of the destination and its culture. That can only enrich your travel experience, and that's why I make an exception.
Some specialize in art and history, others in opera or cooking. There are bike tours of Ireland, wine tours of France, and guided hikes through Sicily. You may ride across Tuscany on horseback, visit Greek ruins with an archeologist, or pay homage to the Shakespearean sights of England.
There's no way I can list all the specialized tour agencies out there. That's what the box to the right is for. Those indicies will point the way to all the myriad specialty tour operators out there, from language classes and cooking schools to Himalayan treks and scuba adventures, with bike trips, archaeological digs, art schools, cultural cruises, and transalpine ski odysseys thrown in between.
The particulars, well, that's what much of this section is devoted to, via the links above.