In Europe, there are several great conferences regarding open source technologies:<p>Open Source Summit: <a href="http://events.linuxfoundation.org/events/open-source-summit-europe" rel="nofollow">http://events.linuxfoundation.org/events/open-source-summit-...</a><p>Euro Python: <a href="https://ep2017.europython.eu/en/" rel="nofollow">https://ep2017.europython.eu/en/</a> (2018 site is not yet, and will be probably somewhere else)<p>Django Con Europe: <a href="https://2018.djangocon.eu/" rel="nofollow">https://2018.djangocon.eu/</a><p>And many cool local PyCon's, such as PyCon SK 2018: <a href="https://2018.pycon.sk/en/index.html" rel="nofollow">https://2018.pycon.sk/en/index.html</a><p>Disclaimer: I am part of the PyCon SK organizational team
What kind of apps do you build in? Those are big conferences, but if they don't apply to you, they aren't any good. (I just attended AWS Reinvent, and it was probably the best conference I've attended, but wouldn't be valuable if I wasn't working heavily in AWS)
This year I went to Rails conf in Phoenix and Ruby conf in NOLA. Both were great. Excellent organization and a good variety of topics (I don't develop exclusively with Ruby/Rails).