The Israeli tax authority recently devised a public campaign to encourage people, general citizens, to call in and report about people who are not paying their taxes - working black. The snitch line has come to be known as the "malshinon" though the official name is "Kav Tzedek" - the line of justice.
The person calling in and reporting someone else does not get anything for his efforts - no payment per name or per x money collected as a result of the phone call. The tax authority's campaign to the public has focused on the idea that those people who do not pay their taxes are sponging off the rest of us, getting all the benefits of society, benefits that cost money to supply, while not paying in their share. Calling in will help the "snitch" by getting those people off the backs of everyone else, they will pay their fair share, and maybe we will all end up paying less if more people are paying in (my addition).
Surprisingly, in the first four days of the malshinon program being active 3000 people have called in to snitch on people not paying taxes. Some callers were complaining about government policy or large corporations that don't pay enough taxes or get their tax levels lowered in forms of assistance, but the overwhelming majority were actually "snitches" informing on others. 80% of the callers were anonymous. The tax authority says they had not expected such an overwhelming response.
(source: Globes)
Interestingly, the people snitched on the most include: landlords renting apartments but not reporting the income, shiputznikim (small renovations contractors), and taxi drivers.
(source: Nana News)
Rav Shmuel Eliyahu, Chief Rabbi of Tzfat, was asked on the radio if halachically one is allowed to call in to the malshinon and report someone. Rav Eliyahu's response was that we are not allowed to snitch. Let the tax authority do their own work, and that is fine, but we do not snitch.
Rav Eliyahu compared it to the story of Achan who took spoils from Jericho despite the ban - Yehoshua asked God who had violated the ban and taken the spoils. Hashem's response was he would not tell him, but if Yehoshua wants to investigate, he can do so. Rav Eliyahu said this story teaches us that snitching is not appropriate behavior.
(source: Srugim)
I dont know if Rav Eliyahu's opinion applies also to someone who stands to benefit directly, though I cannot think of an example of someone who would benefit directly by snitching, rather than just snitching for either revenge, retribution or for no apparent reason.