Slug's Guide to IDL: A great collection of resources compiled by UC Santa Cruz grad students. A few highlights:
- Carl Heiles's Quick IDL Tutorial #1: A great starting place for beginners.
- Carl Heiles's Quick IDL Tutorial #2: IDL Datatypes and Organizational Structures
- Adding programs to your IDL path
- The where() function
- Reading ASCII data (text files)
- Reading FITS images
Plotting examples with the new "object graphics" method
IDL Astronomy User's Library: The best thing about IDL is the huge astronomy codebase. If you think it has already been done, it probably has: don't reinvent the wheel! This is a good place to start looking.
The IDL Coyote, aka David Fanning, will surely show up in many of your Google searches. His website has lots of great question & answer archives, tips, and a code library. The Coyote has a specific page on object graphics.
NumPy for IDL users: Useful mapping between IDL and Python if you use code written in both languages