Where and how should one recommend outside sites?

[KageKamen]

Not pay sites, but something like someone's Deviantart.

 

[Eulalia]

Not pay sites, but something like someone's Deviantart.

Whichever part of the Forums seems most relevant -
CF (Crux) Pics and Videos, BDSM Pics and Videos (or Poems and Stories), or Fantasy Unlimited.
If it's a free, open-access site like DevArt, just post a link and a small sample.