Compiled (and updated) over 5 years. Open to suggestions, feedback and eye-openings,
Here Maps: You can download the maps for most any country, or even the entire world if you have the space, which is great since Google Maps basically sucks for offline usage.
uCiC: For finding things nearby to see, checking out crowds and basically interacting with any human being on the planet :)
Wikisherpa+ (cost $1 I think?): For downloading wikivoyage/wikipedia articles for offline usage. I'm not certain if it's the best app or not, it's just the one I happen to use, but there are others.
Alltrails: For finding/saving hiking trails. I've only used that one for weekend trips where I live though (US), not for traveling other nations.
Eventbrite: This has given me a few pleasant surprises. If you're traveling to a new city, give this a glance and you may find some amazing event you would have missed.