Core Java Refcard
The refcard contains the usual items--keywords, operator precedence, and so on, together with the things that I keep forgetting, such as the exact syntax of MessageFormat strings. (Did you know that you must use two ' for a single quote? I kept forgetting that and finally developed the memorization aid that the ' remind me of the voodoo doll pins that I'd love to use to torture whoever came up with that. Use the refcard, and you won't forget.)
The focus is on those items that every Java programmer needs, whether on the client or server side. Logging. Formatting. Internationalization. Regex. Properties.
What would you like to see on a future edition? And what would you take away to make room for it?