If your research involves immunoglobulins, T cell receptors, major histocompatibility complex or related proteins of the immune system then the IMGT/3Dstructure-DB is worth checking out.

I am a fan of making the PDB more useful and this database fits the bill.

This database helps to reduce performing sequence alignments by displaying amino acid sequences from specific regions. If you have worked with the Kabat books then you will certainly appreciate this resource.

Here is an abbreviated search example containing the CDR3s of various immunoglobulins:

3Dstructure-DB
Also the database will allow you to search by receptor type, protein name, species and more.

The FAQ on the IMGT/3Dstructure-DB is quite good because it explains the terms used in their database, not just how the database functions. This allows a non-expert in the field to use and understand what is going on, thus making their database an even better resource.