"This shall they give - everyone who passes through the census - a half shekel of the sacred shekel, the shekel is twenty geras, half a shekel as a portion to Hashem."
In Parshas Ki Sisa, the Torah instructs every man to give half a shekel (known as machsis hashekel) towards the communal offering given in the Mishkan (Tabernacle). Since the destruction of the Beis HaMikdosh, we no longer merit to have this Mitzvo, however, we remember it every year when we read Parshas Shekalim. Accordingly, there still remain valuable lessons that can be derived from the machsis hashekel.
The Medrash Rabbah offers a surprising reason for the mitzvo, and in particular, why the specific value of half a shekel, must be given. The Medrash explains that the giving of the half shekel is an atonement for the sale of Yosef Hatzaddik by his brothers. The brothers sold Yosef for twenty pieces of silver. This is equivalent to five shekel. Ten of the brothers sold Yosef, each one receiving one tenth of this value, making a half shekel each. Accordingly, since each brother gained half a shekel in the sale, their descendants were instructed to give half a shekel as an atonement. The obvious question to be asked is what is the connection between the giving of half a shekel and the sale of Yosef?
In order to answer this, we need to deepen our understanding of the sale of Yosef. The brothers knew that twelve tribes were destined to come from Yaakov Avinu. Each tribe would have its own unique qualities and they would all join together to combine to make up the Jewish people as a whole, with tribe complementing the others. The brothers decided that Yosef had lost his right to be part of this group, because of what they perceived to be his dangerous attitude and behavior. Therefore, they believed that they could remove Yosef from the destined 12 tribes, and be left with only eleven. The chiddush (novelty) of this approach was that they planned to remove one of the twelve pieces to the puzzle that would constitute the Jewish people. They felt that they could do without Yosef's potential contribution to the Jewish people, and the Jewish people could continue without him.
With this understanding we can now explain how the mitzvo of Shekalim atones for the sale of Yosef. The commentaries note the significance of the fact that one must give half a shekel as opposed to a full shekel. Many explain that it comes to teach us about the importance of unity amongst the Jewish people by showing that each person is only 'half a person' without combining with the strengths of his fellow man. One should not think that he can separate from his fellow Jews and be unaffected. A person who ha this attitude he will be incomplete. In this way, the mitzvo of giving half a shekel can act as an atonement for the sale of Yosef. Yosef's brothers thought that they could get along fine without Yosef's contribution to the Jewish people. Their mistake was that even if they believed him to be erring, he was still an essential part of the Jewish people. By giving half a shekel we remind ourselves that this is not the correct attitude - all Jews are part of a unified whole, and everyone needs to combine with their fellow.
This idea even extends itself to people who are not behaving in the most optimal fashion. Shortly after the mitzvo of giving half a shekel, HaShem commands us to combine a number of spices to make the incense. One of these is the chelbanah, which Chazal tell us has a foul smelling odor. Why then is it included in the ingredients for the incense? The Gemara explains that any communal fast that does not include sinners is not considered a proper fast. Rav Chaim Shmuelevitz zt"l explains that when the Jewish people are not united, then they are not considered one unit, and therefore the power of the community is drastically weakened.
The Bostoner Rebbe zt"l epitomized the attitude that every Jew should be treated with respect regardless of his religious affiliation. His funeral testified to this by the fact that there were numerous people attending who would not be classified as regular Bostoner Chassidim. He expressed his attitude in this area in a couple of brief sentences: "The trouble with our generation is we only love our fellow man if he's like us - if he davens in my shtiebel , if he has the same Rebbe, if he goes to the same yeshiva - then [he says] 've'ahavta lereyecha'. If he's not 'camocha', then I have no business with him." In a similar vein, he said, "When people try to disassociate one group from another, that's part of the 'torah' of sinas Yisrael (hatred of Jews). Every person can improve. Every group can improve. But it doesn't mean that these people have to be blackballed because some people think that they're not exactly the way they are..."
We have seen how the Medrash connecting the episode of the sale of Yosef to the mitzvo of giving half a shekel, teaches us that we should realize that we should never 'blackball' other Jews, regardless of who they are. May we all merit to learn from the words of the Bostoner Rebbe zt"l and emulate his actions, in striving to unite all Jews.
 Ki Sisa, 30:13.
 Bereishis Rabbah, 84:17. With commentary of 'Matnos Kehunah'. One may ask, that according to this reasoning, the descendants of Yosef and Binyomin (who was not involved in the sale) should be exempt from this Mitzvo. It seems that there are other reasons for the Mitzvo of Shekalim which obligate every man to give it, however the amount of half a shekel is fixed by the calculation made by the Medrash.
 See Tallelei Oros, Shemos, chelek 2, p.202 in the name of the Chida, and beshem amroo, Shemos, Ki Sisa, 30:13 in the name of Arvei Hanachal (author of Levushei Srad on Shulchan Aruch).
 Krisus, 6b.
 Sichos Mussar, Maamer 54, p.231.
 This is a term used for a place of prayer.
 Quoted in Mishpacha Magazine, Issue 287, 22 Kislev, 5770, p.41.