Did Freya's necklace have special powers to it? She wanted the jewel so much, I've heard she bedded several dwarves in order to have it, so it must have been unique.
No magical powers.
At least none that are recorded. The Brísingamen is only named a few times: In Þrymskviða in the Poetic Edda, where Thor is disguised as Freya and wears it, and by Snorri, who uses it as a kenning for Freya, "owner of Brísingamen" (Skáldskaparmál, 20) and Loki, "thief of Brísingamen" (Skáldskaparmál, 16). There are a few more texts that alludes to some necklace of Freya's, but they don't positively identify it with Brisingamen.
Chief of these is the Sörla þáttr, a short story in Flateyjarbók in which we are told that Freya vwanted to buy a necklace from four dwarfs, and that they refused any other payment but that she should spend one night with each of them. As the Flateyjarbók was compiled by two priests, and the story is in other ways quite critical of Freya, it is a bit unclear to what extent the details should be believed. It does go on to say that Loki stole the necklace on behalf of Odin, and that he had Freya perform a magical feat to get it back.
This ties in with another, much more fragmentary story in the poem Húsdrápa, which we have through Snorri, who cites it in Skáldskaparmál (part 16). It tells of how Heimdall and Loki, in the sahpe of seals, fought over "hafnýra fögru". Directly translated, this becomes the "beautiful seakidney"; hafnýra also appears to have been an amulet worn by women for easier birth. A kenning for Heimdall (Skáldskaparmál, 8) recorded by Snorri calls him "The retriever of Freya's jewellery".
And that is really all there is that are told about Brísingamen in Norse mythology. The latter half, "men", is a word for a torque or necklace, so it seems the name means "necklace of the Brisings", which is supported by a mention in Beowulf (line 1199) of Brōsinga mene. As it is a dwarf-made necklace, we can suppose it to be quite superior to most jewellery, but there are no mentions of any special powers.
Apart from the sources quoted above, I've found Britt-Mari Näsströms Nordiska gudinnor useful, as Well as Gro Steinsland, Fornnordisk religion.