For all the comments about crackmes and patching around messageboxes, I would like to mention that reverse engineering is arguably more valuable for its ability to surface file format information and/or protocol specifics.

In several countries, this is one of the expressly granted rights with respect to reverse engineering since it promotes interoperability and (more often than not) allows one to use a file format (or network service) long after the original software has expired [in the mortality sense].

I also highly recommend checking out the new http://reverseengineering.stackexchange.com/ since it is currently in its beta period and needs all the content it can get.