Depending on your point of view, single supplements are either unavoidable and fair, or else the worst act of perfidy in the travel universe.
To view it from the positive perspective, most hotels these days do not charge extra for two people sharing one room. If I'm traveling with a companion, this pleases me!
But, from the single traveler's point of view, that $150/night hotel room, which splits to $75 each for two people traveling, becomes, all of a sudden, twice as much for the single traveler.
There is more to a single supplement calculation than just doubling the per person room costs. If that same hotel also included a free breakfast, then it should charge less for a single person (in total) than for two people, because it is only providing one breakfast. Some hotels that have various 'per person' type inclusions within their rates will recognize this fact and adjust their single supplement accordingly, but others will not.
If you are on a multi-day tour that includes accommodation, then you'll find these single supplements appearing in the tour price as well, because obviously the hotel passes such costs on to the tour operator. But because some of the tour inclusions are 'per person' items like admissions, the seat on the coach, air fares, etc, the single supplement on a tour is not (or at least should not be) a doubling of the 'per person share twin' rate.
However, some (very greedy!) tour operators will say 'because you have deprived me of the chance to sell a companion onto my tour, I'm going to make you (the single traveler) give me all the profit I would have made from two people traveling'! Stay well clear of operators that take this attitude to single travel.
However, no matter how it is calculated, and how much it is, the cost of a single person traveling alone should always be no more than the cost of two people traveling together.
Some hotels and some cruise lines have special 'single' rooms at rates substantially below twin room rates. These rooms are sometimes much smaller than a twin room, with only one single bed in them, but sometimes the 'single room' is purely a marketing concept, and single people will get exactly the same room, for much less money, than a couple would get if traveling together.
These enlightened hotels and cruise operators understand that if they have a definite number of empty unsold rooms or cabins, they are better advised to sell them at a lower price to single travelers than they are to leave them empty and unsold.
How do you find such hotels and cruise lines? The best approach is to use a specialist travel agency that deals with singles travel.
If you're traveling alone, you should turn the 'single supplement' issue around on its head and ask for a 'single discount' off hotel rates. These seem to be more prevalent in Europe than in North America, but it never hurts to ask, wherever you are, and it might encourage the hotel to give you some type of discount, even if it isn't officially a 'single' discount.
Some regular tour operators have a 'twin share' program where a person that is traveling alone need pay only the regular 'per person share twin' rate . The tour company then undertakes to match that person up with another person of the same gender who will share the room. If it is a guaranteed program, and the tour operator fails to find another single traveler on the same tour of the same sex, you get to have a room to yourself without having to pay the single supplement - a great deal.
Some tour operators will only offer to match you with a person of the same sex. Others will also match you with a person that is either a smoker or nonsmoker, based on your own preference.
But in such cases, you are still a single person on a tour that is mainly couples. Hopefully your room mate turns out to be a good companion, so that you can enjoy both someone's company and also the lower cost of the twin share rate.
Many of the specialized singles tour operators operate dedicated singles-only tours (or block a large number of cabins on cruises and maybe even have their own dining area), and in such cases, most people choose to share with another traveler (although single rooms are usually available as an extra-cost option).
Singles touring is sometimes offered to specified age groups (eg 18-35, or 30+) and sometimes the tour operator tends to generally get a fairly clearly defined demographic of attendees. You should ask to understand the likely type of people that you're probably going to be traveling with in terms of age and even nationality. Most companies are coy about detailing the mix between men and women on these tours, but it seems that it is more common for there to be slightly more women than men in most cases.
Some of the singles tour operators concentrate on the travel experience alone. But some of them add the possibility of a romantic element, by pitching their travel more clearly to single people that are looking for partners, not just for a short vacation, but for their future life as a whole!
Several of the operators listed on the links page do not arrange any touring at all. They simply help you to meet a like minded travel partner, either of the same or opposite sex, and with or without romantic overtones, and then, after the two of you have got to know each other, you're then free to make whatever travel (or other!) arrangements that you might wish.
There also exist services for assisting travelers with special needs. One excellent example of this is the Journeywoman website that helps solo woman with the travels. Other sites exist for disabled travelers and gay travelers, and probably for just about every other category of traveler too.
Several of the operators on the links page have free e-zines that they send out every month or so. If you find an operator that appeals to you, be sure to put yourself on their mailing list.