There are numerous forums for Poser, Modeling, Art and all manner of advice for your artistic pursuit. Below are just a few of them that I frequent;)

Koshini Alerts Mailing List - This is the mailing list that I and other Koshini developers use to alert folks of new releases. It's not the end all of lists but people spotting new stuff are encouraged to post even if they aren't the maker ;)

Koshini Forum - This is a Koshini & Friends only forum for those of you wanting to show off your renders, seeking help, seeking special pictures made... You name it, just so long as its Koshini & Friends ;)

Plushies Forum - This is the Plushies Forum, perfect for keeping up with all things new in the plushie world, seeing new pictures by fellow Plushie users and requesting new things for your plushies!

RuntimeDNA - A Poser Store and a user/learning center for Poser as well as other software.

Renderosity - A Poser Store with lots of tutorials as well. HUGE Number of freebies.

FaerieWylde - A Poser Forum specializing in faries and fantasy. Attached to store FaerieDreams.

Daz3d.com - A Poser Store. Will have to hunt for the forums and tutorials but they are there.

CGTalk - A pure 3d art forum. Lots of inspiration. Not always the best place to ask newbie questions as they tend towards some snipey answers regarding 'go read the manual' but for finding answers to actual techincal questions they're really really good.

Zbrush Central - Forum full of tutorials free tools and all manner of community support. Newbie or pro questions they're a great place to go for inspiration and to see what's new and what's being developed.

3dTotal - Home of the incredible texturing CDs, A fantastic version of the Joan of Arc Tutorial and a forum to boot.

Silo - This is the software forum for Silo specifically. Very useful.

UVLayout - You'll have to wind your way to the forum but this software and the tech support are WELL worth it.

Righthemisphere - I really don't recommend this forum for anything more than newbie questions. Speaking frankly I've had a confirmed bug report waiting for fixing on Deep UV since December of 2004... But for answering newbie questions they're pretty quick.