Windows: You probably don't send the same exact resume for every single job opening, but keeping track of what you sent can be a confusing pain. ResumeTracker does a smart job of organising and labelling your multi-versioned resumes for later reference.
The app itself is still in its early stages, and there are two things to note right off the bat — the app really requires Microsoft Word, unless your non-Word editing tool happens to agree with it, and the app also requires Administrator access to run well. Those complaints aside, ResumeTracker works just fine for the average Windows and Word user, providing an organised way of creating a Master Resume, creating new entries for each job or job title, then linking a customised version of that master resume to each job pitch, with notes on when you sent it, who it was sent to and the like.
ResumeTracker is a free download for Windows systems only. What's your preferred multi-resume tracking system, other than remembering to rename your .doc files for every single job?