volveré y seré millones
Since this forum is about WADs, and projects in production, it's not uncommon for people to reply with a post where they include a demo recording on any WADs being discussed.
Instead of using some file hosting site that offers a temporary service, with delays, CAPTCHAs or ads, don't be afraid to post the demos in the Doom Speed Demos forum, where you can directly attach demos. Make your post normally, here on WADs & Mods, and use the Demos post in the way you'd use a file hosting service. For clarity, you can provide links between each post so the people browsing the WADs & Mods thread can easily get to the demo, and those in the Demos thread can read where the demo is coming from.
You can post such demos in the Miscellaneous speed demos, Miscellaneous first attempt demos or Miscellaneous TAS demos threads, any suitable thread with demos for the WAD in question, or by making a new thread. Use the miscellaneous options if it doesn't seem like the WAD will produce too many demos, and the last option if it's the kind of multi-level WAD that will attract speed runners.
In some instances, especially test demos for betas of WADs that will likely be replaced later, you might want to resort to one of those temporary sites, such as Rapidshare or whatever, if your demo is meant to be only temporary. Otherwise, people will be left with a demo and no WAD to run it with.
For more information on attaching demos on the Doom Speed Demos forum, see the attached threads, especially the basics here.