I've been searching the web for freestanding (not drilled into the ceiling or hanging from rafters) ring hanging solution possibilities that I could use in my condo. Most of the ideas/equipment listed below could be used indoors in an apartment, although some are outdoor rigs. Feel free to add to the list or if you own or have used any of the equipment comment on it's stability, strength, etc. Hopefully the links are fairly stable.
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HOMEMADE/CUSTOM EQUIPMENT
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Custom metal solution
http://gymnasticbodies.com/forum/viewtopic.php?f=10&t=911
Wooden frame, outdoor but could be used indoors
http://bodyweightculture.com/forum/showthread.php?t=10248
This site has lots of ideas for inexpensive homemade equipment.
Including 2 freestanding chinup bars, plus a rowing station.
http://www.angelfire.com/ny5/shenandoah/Grunt/Bodyweight.html
All wood pullup system
http://img329.imageshack.us/my.php?image=pix10tp1.jpg
http://www.rosstraining.com/forum/viewtopic.php?t=28980&sid=05b8ec8f06f4685f1df472038b6130cd
Someone brainstorming a multi person pullup station.
http://board.crossfit.com/showthread.php?s=a762b177f47ad46d1e7d52bc8b0a6d8f&t=26373
One response includes a link to a welded metal solution
http://faustin.livejournal.com/274463.html
Another response shows an outdoor multi ring setup made of wood
http://board.crossfit.com/attachment.php?attachmentid=1340&d=1202678480
Homemade power cage from wood and pipe
http://www.home-gym-bodybuilding.com/homemade-power-rack.html
Discussion around building a homemade power cage
http://forum.lowcarber.org/archive/index.php/t-138781.html
A variety of homemade equipment ideas including a wooden power cage
http://www.davedraper.com/pmwiki/pmwiki.php/PmWiki/HomemadeEquipmentIdeas
Outdoor metal frame system
http://crossfitlp.com/200810/pull-up-bars/
Crossfit link to other links on pull up bar(s)
http://board.crossfit.com/showthread.php?t=32564&highlight=power+cage
Wooden solution (wall presumably provides some lateral support)
http://www.youtube.com/watch?v=SM7gjO2ovVI
Article about creating your own home gym...
http://www.crossfit.com/journal/library/cfjissue1_Sep02.pdf
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COMMERCIAL EQUIPMENT
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Freestanding, adjustable chinup bar (300lb capacity)
Height adjustable metal chinup bar (perhaps the same system but different vendors?)
http://www.torqueathletic.com/ts1/products.php?prod=MA03-1
http://www.mbodystrength.com/taatpusy.html
http://www.tacticalathlete.com/tapsbar.htm
A couple of power cages
http://www.treadmillfactory.ca
Portable chipup bar (with no floor cross piece) (350lb capacity)
http://www.amazon.com/All-one-Stand-Alone-Pull/dp/B001KUURTS/ref=pd_sbs_sg_8
http://www.trapezerigging.com/PortablePullupBar/
Portable chinup bar (with floor cross piece)
http://www.amazon.com/Portable-Chinning-Bar/dp/B000BWCID4/ref=pd_sbs_sg_2
http://www.onlinesports.com/pages/I,OLY-BE106M-3.html
Chip bar but it has a cross piece in the middle? (300lb capacity)
http://www.homefitnessequipment.org/modifiedpullupbarfreeshipping.aspx
Lots of power racks, including a 2 pillar power rack
http://www.newyorkbarbells.com/powerracks.html
http://www.newyorkbarbells.tv/96031.html
Power station without a back support
Wooden A frame example
http://www.porchswings.com/stands-accessories/porch-swing-stands/greatamericanaframestand.cfm
Google images on power racks
http://images.google.ca/images?ndsp=20&hl=en&q=%22power+rack%22&start=40&sa=N
Welder in Woodstock (also a powerlifter), makes custom equipment