Updated
If your wanting to know more about MySQL Indexes on tables, then check out Understanding Different MySQL Index Implementations.

I just discovered today in the MySQL 5.1 Reference Manual a handy set of additional indexes in the System Navigation section.

• Index
• Standard Index
• C Function Index
• Command Index
• Function Index
• INFORMATION_SCHEMA Index
• Transaction Isolation Level Index
• JOIN Types Index
• Operator Index
• Option Index
• Privileges Index
• SQL Modes Index
• Status Variable Index
• Statement/Syntax Index
• System Variable Index

Perhaps they have been around for some time and I’ve not noticed, but there are much better then searching when you know the content type as per the index list on what you are searching for.