I've broken the SQL migration resources down in to SQL Server 2012 and SQL Server 2014 lists, because now we are really entering the territory of requirements that are potentially being imposed by the software that is leveraging SQL Server, so upgrading to the latest version may not always be possible.
SQL Server 2012 migration resources
SQL Server 2012 Upgrade Technical Guide
This technical guide takes you through the essentials for upgrading SQL Server 2005, SQL Server 2008, and SQL Server 2008 R2 instances to SQL Server 2012.
SQL Server 2012 Upgrade and Application Compatibility
This wiki is focused primarily on SQL Server 2012 Upgrade And Application Compatibility Testing (AppCompat) and describes how to systematically identify potential upgrade blockers in applications that have a dependency on the SQL Server database engine.
Upgrade Assistant Tool for SQL Server 2012
Upgrade Assistant guides you through the steps required to create a test workload on an instance of SQL Server 2005/2008, upgrade the database to SQL Server 2012, and replay that same workload to identify differences in behavior, if any. In this process, Upgrade Assistant provides automation for backing up and restoring necessary databases, capturing and replaying workload traces and capturing trace results. Finally, Upgrade Assistant compares the trace results and identifies areas where the test workload replay produced different results on SQL Server 2012.
SQL Server 2012 Developer Training Kit
The SQL Server 2012 Developer Training Kit is a collection of content that can be used to familiarize a developer or IT professional with key improvements in SQL Server 2012. Download and install the SQL Server 2012 Developer Training Kit 
to get your own copy of the training kit content. See the SQL Server 2012 Developer Training Kit BOM wiki for a detailed list of the content in the training kit.
Upgrade Assistant for SQL Server 2012: Create a Test Environment
Upgrade Assistant is designed to run in a test environment. Application compatibility testing should never be performed on a production environment. When possible, application compatibility testing should be performed on systems that mimic the configuration of your production environment from both a hardware and software perspective. This is ideal if you are planning a side-by-side upgrade as the test server could then be promoted to production once testing is complete.
SQL Server 2014 migration resources
You can upgrade from SQL Server 2008, and SQL Server 2008 R2, and SQL Server 2012. This topic lists the supported upgrade paths from these SQL Server versions, and the supported edition upgrades for SQL Server 2014.
Upgrade to SQL Server 2014 Using the Installation Wizard (Setup)
The SQL Server Installation Wizard provides a single feature tree for upgrade of SQL Server components. You can also install SQL Server 2014 side by side with an earlier version, or migrate existing databases and configuration settings from an earlier SQL Server version, and apply them to an instance of SQL Server 2014.
SQL Server 2014 Upgrade Advisor
SQL Server 2014 Upgrade Advisor analyzes the configuration of installed SQL Server components, and then generates a report of issues that you must address to assure a successful upgrade.
Planning a SQL Server Installation
This planning document covers the installation requirements, system configuration checks and security considerations to upgrade SQL Server to a later version, and using the SQL Server utilities to configure SQL Server.
Microsoft SQL Server 2014 Migration: Case Study Featuring In-Memory and Hybrid Cloud Scenarios
In this recording, the presenters share their experience upgrading mission critical databases supporting large workloads to SQL Server 2014 with minimal downtime. They then discuss the practical process of enabling some of the new features introduced in the 2014 release including in-Memory OLTP, Managed Backup, and Data Files in Microsoft Azure Storage.
Microsoft SQL Server 2014: In-Memory OLTP End-to-End: Preparing for Migration
SQL Server In-Memory OLTP is a key new feature included in SQL Server 2014. It has been proven to dramatically increase OLTP performance without hardware changes. However, there are numerous nuances in In-Memory OLTP. These nuances mean that not all workloads can derive benefits from In-Memory OLTP. For the workloads that can benefit, care must be taken so that the migration process will enable the workload to take maximum advantage of In-Memory OLTP while keeping efforts down to the minimum. This session uses a sample workload to demonstrate how to plan to migrate to In-Memory OLTP.