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:

The blog is the most versatile and user-friendly place on Multiply. If you have a recent version of the most common browsers (Firefox, Mozilla, Netscape, IE) you should see a toolbar just below the item title that allows you to modify the text style and do other things in your entry. The toolbar looks like this (click to enlarge):



I will explain what the buttons labeled with numbers in the above pic do:

1. You can add a pic to your blog entry. When you click on it, it will display another window where you can choose to import a pic from your computer (upload it), use one from one of your albums (Photos entries) or from somewhere in the Internet:



After that, you should see a box where you can choose the justification: left, center or right. Click on the one you want to use and the image is added to your post.

Tip: if you add an image to the beginning of the entry and align it to the right or to the left, a thumbnail will be displayed in your homepage after you publish the entry if your Blog box is set to 'Summary' view.


2. When you click on the checkbox named 'Edit HTML' you can see the HTML code used in the entry. Then you can fine tune your entry if you want or you can add those widgets from the web, like a slideshow, a movie or even a song player. Sites like YouTube and Slide give an EMBED code that you can use at other places (Multiply does it too: click on the share link found in most posts), and you can add it to your blog entries after you click on 'Edit HTML': just find the spot you want to place it and paste the code.
Another tip: you can compose an blog entry, click on 'Edit HTML', copy the code and then paste it anywhere in Multiply, like in your other entries and replies. Think of it like an HTML editor, where you create your stuff and then get the code to paste where you don't have these handy tools.


3. The paper-clip allows you to add attachments to your blog entry. The file will be listed below the entry for downloading. Great for sharing PDF, PPT, PPS files etc. You can add up to 5 attachments to a blog entry.


The rest of the buttons in the toolbar are self-explanatory, just mouse over them and wait a bit to see what they do, like the icon to the left of the paper-clip is for adding polls to your blogs.


Hope this helps. Happy Multiplying.