Learning

• BuildRoot is a set of makefiles and patches that are intended to simplify the process of building an entire embedded Linux system.
• A sizeable number of source file tarballs are available
  
• Cross Linux From Scratch is the website of a project dedicated to instruction in creating your own customized Linux system from source code
• Documents on Cross-Compiled Linux from Scratch - Embedded:
• ARM
• MIPS
• x86
• Embedded Linux Wiki offers a lot of embedded Linux related information
  

Emulators, etc.

• QEMU is an emulator that can allow code to run on a host system (e.g. Intel-based Linux desktop) that was written for a different hardware platform (e.g. ARM).  There are a number of useful QEMU related links:
  
• QEMU ARM system with DirectFB interface
• Getting started with an embedded Linux system emulator
• Tutorial: a simple embedded Linux system
• System emulation with QEMU
  

Packages

• TSlib is a hardware abstraction layer (HAL) for touchscreen hardware.
• Wiki page of information, offered by Texas Instruments
• Download link for source tarball
• Github repository of source; 

References

• Choosing free software graphical libraries for embedded devices, a PDF of the presentation brought to the 2008 Embedded Linux Conference Europe by Thomas Petazzoni with Free Electrons  (eLinux also has an older copy of the PDF document)