On package vacations, single travelers are often hit with a "single supplement" to the base price. To avoid it, you can agree to room with other single travelers or find a compatible roommate before you go, from one of the many roommate-locator agencies. This is also a good way to avoid the relatively high cost of a double room-single use as opposed to splitting a double.
Travel Buddies Singles Travel Club (tel. 800-998-9099; www.travelbuddiesworldwide.com), based in Canada, runs small, intimate, single-friendly group trips and will match you with a roommate free of charge. TravelChums (tel. 212-787-2621; www.travelchums.com) is an Internet-only travel-companion matching service with elements of an online personals-type site, hosted by the respected New York-based Shaw Guides travel service.
Many reputable tour companies offer singles-only trips. Singles Travel International (tel. 877-765-6874; www.singlestravelintl.com) offers singles-only escorted tours to such places as London, Alaska, Fiji, and the Greek Islands. Backroads (tel. 800-462-2848; www.backroads.com) offers "Singles + Solos" active-travel trips to destinations worldwide.
For more information, check out Eleanor Berman's classic Traveling Solo: Advice and Ideas for More Than 250 Great Vacations, 5th Edition (Globe Pequot), updated in 2005.