Thee are a lot of them.
http://www.justrightcarbines.com/ - looks and some ergonomic similarity to AR-15
Lone Wolf has a Glock lower receiver for AR that takes glock mags
Many 9mm AR-15 variants that take Glock mags
There have been mag well blocks produced at various points that reportedly worked well.
MSAR was working on some options and mayb produced them, but they recently shut down their firearms section and left their firearms customers high and dry.
The old Keltec seemed to be the best unless you wanted to spend close to $1000 IMO. I have wanted one for a while but they have always been a little hard to come by. Available when the market is calm, but that always seems to be when cash is tight. With their recent Gen II redesign their production is supposed to double and all the reviews indicate across the board improvements with no steps back. Hopefully I can snag one of the new ones when the price levels out or pick up one of the old ones at a discount.