One of my favourites, I suppose. I refer to the collection of rules on my bookshelf. Many I have played once (some - like Flames of War - I have never played), while others get barely a look.
But its not just rules sets. Its the assorted stuff that comes with them. Field of Glory comes to mind. Now I like FOG. I bought the Rise of Rome book and - if I'm honest - not even scratched the surface of that period (unless you count the 20 odd Punic Wars battles I've had with Russ).
So what prompted me to then buy the expansion packs for the Greeks (I only have the Macedonian army at present), Imperial Rome (no Imperial Roman army at all), Oath of Fealty (no armies at all), Storm of Arrows (nope), Wolves from the Sea (ditto)....
Flames of War is another one. I like Spearhead and thought FOW would provide for a faster Thursday night game. I'm sure it will, but I've yet to even read the introduction. Which maked the purchase of three (yes, three) expansion booklets even more unfathomable. Especially as I have yet to paint and base my troops for Spearhead.
I do have POW Napoleonics somewhere - we tried and didn't like. But then bought the Polemos rules - re-based all my Napoleonice to fit the new rule set (never, ever again) - then found we didn't particularly like them. So had to re-base again for Grand Armee.
Some rules we play quite often (like POW 18C and Renaissance) so I bought the rules - but never felt the need to open the books (as the guys I play with are so up with them). So they too are unused.
Maybe its that quest for the ideal rules for each period that makes us do it.