Is there any diff tool for Lotus Notes which allows to compare scripts, design elements and documents?
There is a tool from TeamStudio called Delta: http://www.teamstudio.com/products/delta.html
There is a free tool from OpenNTF which does document comparisons: http://www.openntf.org/Projects/pmt.nsf/ProjectLookup/Compare%20Notes%20Documents
Ytria also has a product which, among other things, will compare data documents (I don't believe it compares design elements).
http://www.ytria.com/website.nsf/WebPageRequest/Solutions_scanEZ_specen
And, I believe Martin Scott (http://www.martinscott.com) has a similar product which compares documents.
If all else fails (and by "all else" I mean the often ridiculous corporate procurement system) you can always do a an export to DXL (or a Design Synopsis for code alone) and use any decent text editor with a diff function. It's not TeamStudio Delta, but it will get you where you want to go.
I had a discussion on my blog a little while back about this:
http://rosshawkins.net/archive/2009/12/24/notesdomino-refactoringanalysis-tools.aspx
However what I've ended up doing in the past is exporting the design to the filesystem and using standard text tools (WinMerge and SublimeText for me personally) to do what I need.
Being able to do the raw dump is something that was added with the Eclipse based designer, and isn't overly obvious, but you can read more about it here:
rosshawkins.net/archive/2010/01/20/searching-the-contents-of-notesdomino-design-elements.aspx
(link mangled as my rep is too low to post 2 links in one post yet!)