I've made a cell for my mod which I'd like to use in another mod. Is there any way of transferring the cell? I tried copying the content of the cell but when I open the second mod there's nothing to paste. Is there a tool or a method that allows me to do this?

Delete everything but the cell you want to copy from your mod with TES4Edit, then merge the mod (that now only includes the cell) with your other mod. Maybe there is a better solution, but I think it should work if you do it that way.

Better solution?

Open the mod you 'want' the cell copied to as the 'Active Plugin'. Also check the mod your copying the cell 'from' so it loads also.

Once its done loading, Rename the cell you want copied to the new plugin.
Then go to the cell and select everything and 'move' it slightly (you may not have to do this part)

Now save your 'active' mod. Quick the CS.

Load the CS back up and only load the mod you copied the cell to. It should now be there.

You can do the same thing for objects, weapons, statics, etc..

This is certainly the easiest method. Just make sure that none of the objects in the copied cell use things that are unique to the origin mod.