Notice that this tricks may not work for you at some moments, since it depends on the server's running software. However, most ed2k servers run the latest version of lugdunum, so this tricks will work for 99% of your servers.
Actually, what the server does is read an index with all files all clients are sharing and see, out of those sharing the file with has value <hash>, which are the most popular files. Low availability files aren't listed.
You can combine the above tricks, so someone could try to search something like related::<some_hash> AND Video AND SIZE > 1000000
The server checks if the file is known.
Search for file except extension ...
You can use the not <query> and !<query> format in the "extension" field too in the search window.
This way you can search for files not containing the given extension, which is often very useful.
The special 'Ñ' character
Current server and client software support unicode so it is no more an issue, but older versions would not support non-english characters, such as the ñ spanish character.
As a solution, the ñ character was aliased to n. So, searching for españa or espana would give the same results.
This aliasing applies also to unicode-supporting clients and servers. The only thing you should notice is that in this case, since ñ is a different character than n and unicoded recognizes it, searching for words containing n will display results containing ñ, but not the other way round.
Search for hashes or exact file
If you want to search for any file which's has value is <hash>' (where <hash> is any MD4 hash value), you can search for edk2:<hash> and you will get the results.
As an extension, if you want to search for an exact file (maybe you want to see its availability or its rate) and searching it by its hash value gives several non-equal files, you can narrow the results by searching by the file's hash value <hash> and size <size>: ed2k:<size>:<hash>
Or even simply the file's ed2k link (anything after the file's hash in the link will be ignored): ed2k://|file|<name>|<size>|<hash>
Default operation: AND -> word separators: , ; . : - _ ' / ! (space)
and & or not
Ability to perform exact searches : Clients can enclose words in ' . Examples : 'blank & john' OR 'the the'
Double-quotes can be used to interpret ( ) (brackets) as normal characters instad of boolean modifyiers