JavaScript (they have the
script tag in the code) and
iframe codes
aren't allowed in Multiply for security reasons. Any other code that is simple HTML or Flash, like the ones that use the
EMBED or
OBJECT command, should work fine. Usually the site give you options for the code: look for the one in simple HTML (no
script tags) or one that uses Flash.
As a side note, sometimes when you try to copy and paste excerpts from other web pages to yours, usually blog entries, you may get the message that 'JavaScrit/CSS isn't allowed', and you can't save your post. It happens because some of the page code was also copied and also pasted with the text. To avoid this, first paste the excerpt in a plain-text editor, like Notepad on Win or Smultron on a Mac, and then copy it again and paste on your page. You will get only the text and should be able to save your entry.
PS: JavaScript isn't the same as Java. Java widgets should work too, as they usually use the EMBED and/or OBJECT tag.
Update1:
EMBED codes that have
allowscriptaccess won't work anymore (they will be disabled when added to Multiply pages) due to security reasons too. Quote from a Multiply staff member:
We disabled script access across the board for all embedded flash applications because it poses a significant security risk. Embedded flash players, with allowscriptaccess set to true could potentially execute arbitrary javascript on the page and send other users' personal information to a third party. 
