HT: Craig Cobb, assuming you didn't mention hemp just because it is a Renewable Resource. Based on what I'm reading, today, I'd say the timing upgrading one's cache couldn't be better.

Consider several ropes for different purposes. Hopefully we will soon need something that doesn't weaken from repeated use, that can be machine-washed, has minimal stretch, allows knots to be tied and untied easily, and that-- while readily available these days-- may need to be conserved in the future as the economy weakens further. Avoid the rush.

Synthetics: http://www.cncphotoalbum.com/rope/

Technical info: http://www.alpineexposures.com/pages/faq-climbing-ropes-explained-test-uiaa

Rappel rope: For many years, traveling and at work, I carried a 200', 7mm, nylon, "static" ("dynamic" rope is designed to stretch and absorb energy) line, in a light color sheath to help reflect UV, that I kept in a relatively light-proof, dust-proof bag and both some hard anodized locking caribiners and figure eight descender. This was strictly for rappelling and I was very careful not to expose it to solvents, high heat, and-- of course-- ultraviolet light.