A list of tested and really helpful components. If you have more that you want to share please leave a comment

GUI/Forms

Acr.UserDialogs

Allows for messagebox style dialogs to be called from your shared/PCL/MVVM code
- Action Sheet (multiple choice menu)
- Alert
- Confirm
- Date
- Loading
- Login
- Progress
- Prompt
- Toast (Snackbars)
- Time

On android, be sure to call UserDialogs.Init() before using!

FFImageLoading

Xamarin Forms plugin to load images quickly and easily on Xamarin.Forms. But also for native iOS/Android

- Drop-in replacement for Forms.Image
- Xamarin.Forms - iOS / Android / Windows (WinRT, UWP) support
- Configurable disk and memory caching
- Deduplication of similar download/load requests
- Error and loading placeholders support
- Images can be automatically downsampled to specified size (less memory usage)
- WebP support
- Image loading Fade-In animations support
- Can retry image downloads (RetryCount, RetryDelay)
- On Android transparency is disabled by default (configurable). Saves 50% of memory
- Transformations support built-in and custom native platform ITransformation implementations.

Ensure you call CachedImageRenderer.Init() on each platform!!!

SlideOverKit

Easily Create Awesome Slideover's in Xamarin.Forms

FreshEssentials

FreshEssentials for Xamarin.Forms has ONLY the most common elements you need for Xamarin.Forms. It’s contains the elements you need in almost every project and nothing more, things like BindablePicker, SegementedButtons, InverseBooleanConverter, TappedGestureAttached, ListViewItemTappedAttached and not much more. It’s the lightweight essentials

XamSVG (if it wasn't so expensive)

Really powerfull SVG Library that supports click commands and bindable color replacement

Device Abstraction

PCLStorage

PCL Storage provides a consistent, portable set of local file IO APIs for .NET, Windows Phone, Windows Store, Xamarin.iOS, Xamarin.Android

SQLite.Net.Async-PCL

SQLite.Net PCL is an open source, minimal library to allow .NET and Mono applications to store data in SQLite databases.
This is a fork of the original sqlite-net project, which aims to cleanup/improve the code and deliver the package as PCL assemblies with some additional platform-specific implementations.

 Xam.Plugin.Connectivity

Get network connectivity information such as network type, speeds, and if connection is available. Additional functionality includes the ability to ping a specific host and port number. Ensure you have proper permissions set by reading the README.

Xam.Plugin.Media

Take or pick photos and videos from a cross platform API.

Xam.Plugin.Version

A cross-platform way of getting your app version.
Use: CrossVersion.Current.Version;

Networking

jose-PCL

Portable Javascript Object Signing and Encryption (JOSE) and JSON Web Token (JWT) implementation for Windows 8, Windows Phone 8, Windows Silverlight 8 and .NET framework platforms
based on PCLCrypto portable crypto layer

refit

The automatic type-safe REST library for Xamarin and .NET

 Helpers

 PropertyChanged.Fody

Never write INotifyPropertyChanged stuff anymore 😀

Polly

Polly is a .NET 3.5 / 4.0 / 4.5 / PCL library (Profile 259) that allows developers to express transient exception handling policies such as Retry, Retry Forever, Wait and Retry or Circuit Breaker in a fluent manner.