Jem and Other Dolls

This is a baby section, just barely beginning since I am not really a collector of dolls other than Jem.  But there are a few items out there that could conceivably be mistaken for Jem items, mostly because Hasbro "shared" their Jem molds with other dolls, and in some cases, Jem dolls/items were cast from other dolls molds.  So far there are a few dolls that I can say for sure have a link with Jem.  So by all means let me know if you have any information that would pertain to this site.

Barbie & the Rockers (Mattel, c. 1986)
Darci (Kenner c. 1978)
Maxie (Hasbro 1988-1990)
Moondreamers (Hasbro, 1987)
My Little Pony (Hasbro,1982-present)
New Kids on the Block Dolls (Hasbro, c.1989)
Rose Petal Place (Kenner c. 1984)
Sindy (Pedigree/Hasbro, 1986-1988 era)
Spectra (Mattel, c. 1987)

The Ken-ization of Rio! (A link donated by Corey!)
Jem Wannabe Dolls

Barbie and The Rockers; Barbie and the Rockers was Jem's main competition.  Jem had about 2 years worth of pre-production before Hasbro officially launched the Jem line. Rumour has it that Mattel caught wind of the secret rock-star doll Hasbro was planning, and rushed out Barbie & The Rockers in 6 months. They even got their dolls out on shelves first!  Though Barbie ultimately (and I think inevitably) prevailed as the reigning fashion doll, I think we can safely say that Jem easily eclipsed Barbie and The Rockers - just try looking up how many Barbie and The Rockers webpages their are, for example.  Let me know if you find one good enough to link to and I'll put it here.  I'm still looking for a half decent one that's not done by a Jem fan ;)
Needless to say, both being rock star dolls from 1986, they have similarities -in fact many newbies mistakenly call Jem dolls Barbie and the Rocker dolls.  One of the good things to come out of this is that Jem can "borrow" some of the Rocker things, for example the Rockers came with a more standard drum set that some Jem fans prefer. There are also costumes that will stretch to fit Jem dolls. 
Dress from Barbie's friend Dance Club Kayla from 1989

Looks a teeny tiny bit like the dress from Let the Music Play.

Thanks to Chlokie for the photo.
Thanks to Rebecca for the information!

Moondreamers
 
Moondreamers came out in 1987, by Hasbro. Their stands were most obviously influenced by Jem.  Some of their tights were also made of the same material as Super Stars doll's socks, as well one had tights made of the same material as 1st Edition Aja's.
My Little Pony
Most obvious is the Rama Llama connection.  There's not much doubt that Rama Llama was made for Jem's third year, to be intended as a mail-in offer.  But after they made thousands of these llamas only to find the line had been cancelled, they became (in Canada at least) "My Little Pony Llama Friend," a special mail-in offer for My Little Pony fans.  I'm sure that My Little Pony collectors are about as baffled as we are by Rama Llama.
A blue Danse radio.  While it did not come with the American My Little Pony outfit shown above, (that one was green) it did come with the UK version of the same outfit.  Thanks Pika for this confirmation!
In 1989, "Rockin' Beat" My Little Ponies came with brushes like this- notice the similar gold cord idea that was used on Jem guitars earlier.  There is also a My Little Pony fashion that features the same material as Roxy's pants.
New Kids on the Block

Information thanks to Leon, noting differences  between a "Joe" doll and Rio.

-"Yes, they do use black Jem mics, but they have part of them cut off at the bottom, plus the head is slightly shorter than on the Jem mikes.
-The bodies are similar to Rio (Sculpted-on underwear included), with major differences being that the NKOTBs have much smaller heads (Without the neck joint- They twist on like Ken's), molded hair, shorter torsos, fixed waists and arms (They don't turn or bend like Rio's), and more defined stomachs (Not exactly G.I. Joe ripped, but more cut than Rio, and even much more than the Eunuch in a Box, b.k.a. Ken)
-Though their bodies are supposed to be close in proportion to Ken's, their feet are about the same size as Rio's, meaning that Rio can wear NKOTB shoes. NKOTB's shoes are a bit narrower, but they still fit pretty well!
- Speaking of NKOTB clothing and other dolls, while Rio can (barely) fit the shirts, the pants actually fit better on Jem!! Yes, and the jackets and sweaters, too!!!  Basically, the NKOTB line does use a body similar to Rio, but more proportioned to Ken. They also look nothing like the real-life counterparts, but that's another story!!!"

Rose Petal Place
These were 5" flower-themed dolls made by Kenner, roughly the same time as Jem. The only real link I have right now is the following:
 
The fabric used on this Rose Petal Place fashion was used on the prototype for Splashes of Sound.  The fabric was not used for the produced fashion however.  I'm not sure which came first.  The Rose Petal Place doll, "Orchid", came with a pantsuit made of the 1st Ed. Aja skirt snakeskin print.
Thanks to Pete B. for providing the photo of the Rose Petal Place fashion.
Thanks to tommy/ng.59 for the photo of the Orchid Doll. 
Spectra
Chrome colour-bodied dolls with tinsel-filled pastel hair and lingerie-like wardrobe... from space!
Link to Modcolors Spectra page.

I'm always looking for more pictures of "Jem-like" toys.  Feel free to send me pictures or information that you think should be featured here.