Like many sailors, I never seem to throw away old lines. State-of-the-art Kevlar cordage that once served as a low-stretch halyard may become a belt to hold your trousers up, but it's never simply discarded with out being re-purposed at least once.
Securing the Boom. Some folks like boom gallows. Some simply crank the mainsheet against the topping lift. I've always favored a short brace line from the end of the boom to a handy point on one side, creating a triangle. The boom holds still, becomes a firm resting point, wear is reduced, the the topping lift is less strained. It is also light and fast. This one was tied from one of the original jib sheets from my 1979 Stiletto during our delivery trip in; a gale was predicted the next day and I wanted everything tight.
I simply clip it up out of the way while sailing.
Securing Coils on Deck. Some lead all halyards and reefing lines into the cockpit, but I like a clean cockpit without all those tails. I don't mind climbing on deck in a tempest and I like the simplicity and low friction allowed by working at the mast.A similar line secures the spinnaker bridle tail tot he tramp when not in use.
However, there is still the matter of tails and how to control the clutter. I've seen all matter of nifty gagits for securing coils in the West Marine catalog, but a simple padeye and 2 feet of 1/4-inch line is simpler, easier to deal with in the dark and rain, more durable, and more dependable. This was tied from retired 1/4-inch Kevlar halyard from the same Stiletto.
Davits. Another old halyard (1/4-inch Stayset-X if you must know - too stretchy for racing boat halyards) hoists the tender on her davits. The original owner had stuffed 3/8-inch 3-strand through tiny blocks and the friction was horrible.
I still have my first climbing rope, nearly 30 years old. It has been retired from climbing falls to a secondary anchor rode. Old dock lines have pulled a number of drivers out of snow banks this winter. Old sheets from my new boat, found in the bilge on the way home connect the outer dolphins of my slip to the dock, making single-handed docking simple.
An old Kevlar sheet was converted into a high life line (no stretch).
The trick is not to just horde, but to re-purpose.