Check again the context where I have used the word "plug-in", and see if its meaning was meant to be about "NSAPI plugins" or a more generic use of "plug-in" as in "software add-on".
It helps to use the correct terms.
But yeah OWB for MorphOS does offer some extensions such as "userscript manager". It's greasemonkey-like (javascript to modify webpages on the fly). Very useful stuff, it can be used to perform all kind of useful tricks, such as fixing the ever changing youtube full screen mode.
OWB also has a Web Inspector extension functionality which is very useful when writing your own web pages.
Adblock extension functionality is built-in as well.
Which leaves exactly one extension that I'd like to have, but which isn't really essential: Tamper Data.
As for downloading stuff: OWB does just fine with the Download manager (built-in). I understand OS4 users though, download manager must seem very advanced technology considering OS4 OWB still doesn't include one (?).