R – Controlling drop down lists in Windows SharePoint Services 3.0 Form

You could either build it as a standalone HTML/ASP.Net page and include it as a page viewer webpart, or roll your own WebPart for it. The latter is somewhat more complicated if you don't already know SharePoint development, although tools like STSDEV can take away a lot of the pain.

WSS has a basic UI for setting item-level permission on list items, but they hide that from the UI for document libraries. If you go into Settings->List Settings->Avanced settings for a list, you'll see the options to do pretty much what you're asking for. However, on document libraries, that UI is not available. The settings it drives, though are avaiable via the object model.

You could set those same properties for a document library like this:

SPDocumentLibrary onlyOwnLib = theWeb.Lists["DocLibName"]  
onlyOwnLib.WriteSecurity = 2;  
onlyOwnLib.Update();  

And that should about do it. However, apparently that doesn't really set permissions; it just controls what the user can do via the UI. If they had another interface to the library (like via WebDAV) or list (like via the web services), it wouldn't prevent them from editing items they didn't create. If you want true item-level permissions, I think you need to go the event handler route.

This post from Matt Morse explains it in more detail, and he even wrote a command line tool to set the property (plus the .ReadSecurity property) for lists and libraries.