First the disclaimer, I only know about these sites becaues I found them while googling for information about coins and medals and signed up to be a member much the same as the members of this forum most stuff found on line I think is random chance you search for something specific and end up with something usefull that you were not actually looking for.
I have used both of these sites for a few years now one site I use for all the more common coins in my collection just so that I know what I have the other I used for my main collection which I then sort into themes and sub-collections.
First off is http://www.neocollect.com I think this site is brilliant it allows you to organise your collection as much as you like much like a database for example I collect crown sized coins so at first I was happy cataloguing my coins into one section then I thought it would be nice to be able to look at silver only crowns so I made a sub-folder and put a copy of silver only coins into their own section, and so it went on a section for royal events a section for aircraft and maritme coins etc. etc. As an example of how much you can break down a collection here's my main profile page http://www.neocollect.com/user/
You can keep your collection private or make it public or you just click a button to make a folder public so parts or all of your collection can be kept private if you want
Neocollect also has a community section were you can add your collection so others can see it and you can create your own communities if you want. I've made a cople of communities one for Commemoritive crowns and another for Royal families coins and medals. You can also make slideshows for yourself or for the world in general.
This site also has a swap/sell theme but I have never used that part of the site, it has a research section which allows you to write up notes and articles about your collection.
Each community has it's own forum attached.
A down side to this site is that you have to add your own stuff if you want a picture of your collectable you have to supply them yourself this is not a site that has a massive collection of collectables where you just tick a box to show them in your collection but it is a very adaptable site because the cataloguing part allows for lots of information and it is adaptable you can add anything medals, coins, stamps, tokens. So that was http://www.neocollect.com
The other site is http://en.numista.com this is a click your coin to add to your collection site and you can add your own coin if it is not in the database quite easily. This site does not allow you to organise your collection in any way countries are listed alphabetical and coins by denomination one addictive feature is the dashboard a map of the world which fills in country by country as you add coins makes you want to get a coin just to fill in the white bits.
I personally use this site just to keep track of my coin collection the coins you get in a bulk lot you don't really collect but want to organise to see what you have again as with neocollect this site allows you to show your collection or keep it private.
This site has a swap coin feature I've used this several times quite succesfully members are rated by other members to give an idea of how the swap went all of my swaps were okay but sometimes things go wrong for one reason or another so swapping is at your own risk.
This site also has a forum.
To sum up the two sites my favorite is the Neocollect because I get a great deal of pleasure from organising my collection here in a way that you cannot in the physical world as too many coins and medals fall into to many catagories in my coin trays they are organised by date at neocollect I can organise a collection into as many groups as I like I have also organised my collection by mint and by designer but I kept them private it's just somthing I'm working on at the moment. I have actully wrote this post using the Neocollect article feature. The Numista site is good for general bulk collection organisation with an option to swap you coins with others and has several good features but of the two the best is the Neocollect for for being able to organise your collection so well.
Hope you find this usefull and at least enjoy looking at the coins and medals on the two sites. I've never wrote such a big posting and I'm worn out now so I'm off for a lie down now.