This small addon by William W. Wong really put a smile on my face!
Many of the source files I work with are huge (> 16000 lines), so I almost always need to do an isearch to find the point I need (which is often a pain due to false positives). Also, I often need to temporarily jump to another place in the same file.
Breadcrumbs are a "light" alternative to regular bookmarks, using a trail (or rather, a circular buffer) of anonymous bookmarks across all buffers. I have set up the keybindings to drop a breadcrumb with S-<space> and navigate back/forward with M-C-; and M-C-' respectively.
As a special bonus, a breadcrumb is automatically dropped when using find-tag to make it easy to backtrack after looking up a chain of symbol definitions. Very useful!
The feature is very well documented, by the way.