The Mishna Berura, to the best of my knowledge, is, for the most part, the accepted psak for the Litvish/yeshivish community. Is there a Halachic work that Chassidim generally turn to for piskei Halacha and especially minhagim? In other words, if one wanted to get a general overview of piskei and minhagei Chassidus, what sefer would he look into?
There is a common misunderstanding about the Mishna Berura. Before the war, each community had a Rav and he would make the necessary Psak for his kehilla. The major Poskim for Bnei Lita (Lithuanian Jewry) was the Chayyei Adam and Aruch HaShulchan. In the years leading up to World War II, the European world was shaken up by World War I, and it is in this milieu that the Mishna Berura wrote his Mishnah Berura. It was not popular at all in his time because communities were still largely intact. In the years after World War II, the Jewish world was a wreck. Communities were flung apart, there was no longer a single community with a Rav, but different communities together with different minhagim. In addition there were many Jews with no clear minhag at all. In this context, the Mishnah Berura became the defacto standard as an easy Halachic work on which to rely when the halacha was not clear or a person did not have a minhag.
For this reason, the Mishnah Berura has become the accepted standard as a basis on which to rely. Many Sephardim also study the Mishnah Berura for understanding the Maran Shulchan Aruch and the Beis Yosef. It is also accepted among Chassidishe circles as the standard halachic reference.
The Mishnah Berura often cites the Baal HaTanya. The Mishnah Berura and the Be'ur Halacha are also key in halachic analysis of the Shulchan Aruch Orach Chayim. In many Chassidishe circles the Mishnah Berura is understood to be the starting point for halachic inquiry.
If you're looking for specifically Chassidishe Minhagim. The Netui Gavriel has a good 20 volumes of the Minhagim of Bnei Chassidim. In addition, the Piskei Teshuva which is basically a supercommentary on the Mishnah Berurah also notes all the Chassidishe Minhagim relevant to the Psak of the Mishnah Berurah.
Chabad Chassidim also use Shulchan Aruch HaRav. I would point out that Shulchan Aruch HaRav is a tremendous sefer, and while the Psak may not necessarily be accepted, it is certainly worth the extra time to learn it.