As someone who has worked as a programmer in many different languages (C, C++, Ada, Perl, Visual Basic, Java, etc etc etc), I have always found it hard to go from one language to another quickly at times. I program for months in Ada and then get called to update a Perl script, all while teaching Basic, so my languages all kind of mingled together. I found it quite useful to have quick reference guides handy. Apparently, so did this person, who has created a site devoted to some of the languages out there:
From his site: Throughout the years I have collected a huge pile of documents that deal with almost every aspect of software development. The purpose of this site is to centralize this information and to make it available to everyone who may need it or shares my passion on this subject. There are still a lot of documents in my "archive" that I like to share with you all.
At the moment this site represents only a fraction of what I want to provide, so a lot of work still needs to be done. As a result, you should consider this site as a work in progress. I'll try to keep you updated on the most challenging, powerful and continuously evolving software technologies of the moment. I hope that especially Java and C++ fanatics (like myself) will find some valuable information over here.