True Calendar 8The native WP8 calendar sucks, it defaults to viewing one day, but doesn't fit it onto one screen, it doesn't have a week view at all, the month view is pretty much useless because you can't read the appointments and if you try to click on them or click on a day it switches to the crappy day view.
I can't recommend True Calendar 8 highly enough, it possibly has too many configuration options, and you can only have a maximum of one account each of Google, MS and Facebook, but apart from that it's pretty much perfect and is what the native calendar should have been.
Ocell (Twitter)Ocell is buggy, crashing and refusing to load tweets relatively often, however in my experience, and given my demands, it is still by far the best WP8 Twitter app that I've seen. My demands mainly being configurable, swipeable columns, mainly consisting of my lists, just like Tweetdeck and then Hootsuite let me do.
Flickr Central / Flickr BoothFlickr Central and Flickr Booth are both very good apps. They both allow you to automatically upload images, periodically update the lock screen image from particular pools of Flickr photos and view activity on your photos and your contacts' latest uploads. They also both improve on the manual uploading of images.
Flickr Central has a better photo page and a better page for showing you activity on your own photos, however it doesn't show replies to your comments on other people's photos.
UPDATE Aug 2014: I don't know if it's WP8.1 or what but recently I've been finding FC's auto-upload buggy and I can no longer share photo URLs and the developer is no longer responding to emails.
Flickr Booth has a nice tile for the start screen that can show you recent uploads from your contacts or activity on your photos. It also allows you to specify what album to auto upload from and what date to start uploading them from. However the lock screen image rotates too often IMHO and only uses your 10 most recent favourites.
The official Flickr app sucks, mostly because the logo and title always cover half the screen. Yes, really. It's also missing auto uploading and uploading multiple images from the share context menu.