Wikipedia writes "A patent is a set of exclusive rights granted by a state to an inventor or his assignee for a limited period of time in exchange for a disclosure of an invention". That, in and of itself, is not evil. What is evil is that patents prevent society from using good ideas because patent holders generally do no care to build and sell their inventions as much they hopes to be able to sue the crap out of anyone who makes something remotely similar.
What patents are supposed to do
The reason why patents exists is because someone thought it was a good idea to protect inventors' intellectual right and to ensure that they could benefit from their ingenuity. For example Jimmy who invents a fusion-propelled skateboard in his basement applies for a patent to ensure he can raise money and build a fusion-propelled skateboard factory without getting one-upped by someone who heard of the idea, but already has the money. Patents are meant to give those with less resources a fighting chance at success. Or so we're told.
What patents actually do
Patents are an idea which came from the 1400s or so (according to Wikipedia at least). They guaranteed some people the exclusive rights to sell or trade certain commodities. Back then, the intentions behind issuing patents was not the noble "giving Jimmy a chance at success"; their purpose was to enforce cartels and monopolies. As monarchy fell, so did state-sanctioned monopolies. Capitalism and free market gradually spread, competition thrived, and everyone was happy. However, the definitions of patents haven't changed. Patents are still things that guarantee the exclusive right to sell or trade a commodity. They are not designed to help inventors protect their rights in a market, they are designed to help inventors hog the market for 10–20 years (if not more).[4-5] Or rather, patents define the rights of inventors to be those of monopoly.
Now there are two things at play here:
• Most patents are not filed by "Jimmy the neighbour". Most patents are filed by research corporations and already-established businesses.
• Most patent-holders don't care about turning their patented idea into products, they simply want to be able to sue anyone who tries to, or to be able to sell their idea to someone who wishes to do it.
What these two factors accomplish is that it removes things from the "economic pool of ideas". Let's take two scenarios to illustrate this.
First scenario: Someone holds on to their patents to make money by selling it.
Let's say someone (or some business) holds a patent for a more energy-efficient lawnmower and doesn't do anything with it other than hold on to it in the hopes that someone will buy it from them. Now suppose these energy-efficient are worth quite a lot, say a profit of 20 million dollars per year could be made selling them. Because the potential profits are so high, the company asks 3 million for the patent. However, say the company that is actually interested in making these lawnmowers doesn't have that sort of money. Maybe a compromise can be reached, maybe not. The consequence is that you either pay a lot of money because some guy who doesn't give a damn about turning an idea into something real is hogging the idea, in the hopes that those who give a damn will pay him, or that nothing gets done.
Second scenario: Someone holds on to their patents to immunize themselves against competition.
Let's say you've found the cure for AIDS. This is obviously worth a lot of money, so you could sell the patent for a bundle. However, if you possess two cents' worth of intelligence, you'll hold on to your patent? Why? Because their are hundreds of millions of people whose lives depend on your product. If you have no competition, then you can basically decide at what price you're selling your pills, and you're guaranteed that people will buy them. So even if these pills cost 3 cents each to make, you'll still sell them at prohibitively expensive prices just because you can. Prices are unnaturally high, and consumers pay more than what they would otherwise. Plus the incentive for research is removed, as you have a working solution and no competition to force you to improve your product. Who cares if your pills work only 90% of the time instead of 95%? People will buy them just as much one way or the other.
The real problem, and the solution
The real negative impact of patents is that they prevent society from implementing ideas for very long periods of time, and allow some people to control the market for equally long periods of time. Meaning that inefficient designs and ideas stay in society a lot longer than they should or otherwise would. However outright banning patents would have some negative impact; Jimmy the neighbour not having time to raise money before someone else copies his idea.
The solution to these problems is to drastically slash the duration of patents. This works very well in science. For example, in astronomy, people make proposal to get observation times from telescope (point this telescope at this star for this amount of time for these reasons). If they get selected, then they are guaranteed exclusive access to the data for one year, as they came up with the ideas and thus deserve to have recognition for it. After one year, the data become public domain, and anyone anywhere in the world may analyze it however they like. The advantage of the proposal makers is having a one-year of expertise headstart over other people.
Patents should be like this. Guaranteed exclusive rights for one to two years (depending on the complexity of implementing the idea), after which the idea becomes public domain. If you have an idea, you have one or two years of expertise headstart on your competition, you have your marketing advantage (original brand, inventors, more experience), and you've had time to raise money to turn an idea from concept to reality. People would stop hogging their patents and would stop asking ridiculous prices for them. Those interested in making better things now can, competition is now healthier and society benefits from better products at cheaper prices.
 Wikipedia: Patent
 World Intellectual Property Organisation. "Why are patents necessary?", FAQ. Accessed 14 March 2009
 Wikipedia: History of Patent Law
 World Intellectual Property Organisation. "What does a patent do?", FAQ. Accessed 14 March 2009
 Wikipedia:Canadian Patent Law
 World Intellectual Property Organisation. The International Patent System in 2007. Accessed 14 March 2009
 JPL/CALTECH. "Cassini-Huygens mission FAQ", Equinox mission. Accessed March 14 2009.