In iTunes you can play videos in a seperate window. Is it possible to make that window stay on top? Preferably without any 3rd party apps?
Yes, there is an option under Preferences -> Advanced Preferences, under the separator saying 'Keep movie window on top of all other windows'.
The window may look slightly different than this (I found this example online), but the wording is still the same.
Yes it's a built-in feature and it's in iTunes preferences. Look Under the advanced tab as pictured below:
Make sure "Keep movie window on top of all other windows" is checked.
Along the same lines, you may want to "Float" other applications, such as a browser watching a playlist on youtube or even your facetime window like I do. I use a program called Afloat that works well.