I only recently started my family history quest and have various (captioned) photos & other items regarding people not related to me. Save starting my own web site, is there some place I can upload them for future researchers where they would be easy to find and available for free?
For photos, one simple (non-genealogy-specific) alternative is to explore Flickr or a similar free photo-sharing site. On Flickr, there's even a Genealogy group. You would need to take care with tagging the photographs to make them as discoverable as possible by anyone searching for the individuals concerned.
You would also need to make sure you also had the rights to upload the photographs -- this will depend on how you acquired them. For example Copyright Issues with Content Providers discusses issues around sharing images you download from the web.
For the BMD certificates, since I'm interested too, I did a little research just now by searching for "Unwanted BMD certificates". One of the main hits was http://www.certificate-exchange.co.uk/ who cater for exactly the sitation you mention regarding certificates.
Well I didn't find a definitive answer but the best bet to start I think is http://www.cyndislist.com/lost/ where there are many links related to this very subject. The one central place appears to be http://searches.rootsweb.ancestry.com/share.html which I assume will remain freely accessible but one can't be sure.
I would suggest the Internet Archive as a good permanent online home for these sorts of things. You can include lots of metadata, and easily link to and from other places on the web. The IA is a non-profit organisation that aims at long-term preservation of digital items.
If someone makes a copyright claim against one of your items, it may be taken down — but this is reasonably unlikely with most genealogical material, I think.
The other site I'd consider is Figshare, which is a commercial academically-oriented open access digital archive. Again, good support for metadata and searchability, and your items will be assigned a DOI for easier citability.
Lastly, as another answer suggests, Flickr is a pretty great place for photos. The original file is stored and can be made available to everyone, and there's good metadata (including Creative Commons licencing, with explicit Public Domain too). I'd avoid using Flickr for documents though, as there are other better places for these.