There is a huge chance that this site will make your day if you are somewhat interested in the physics of sound and light, as it is a retailer of vacuum tubes, which are the daddy of transistors and solid-state diodes; though not currently used massively in electronics, vacuum tubes have several applications in high-frequency radio transmissions, amplification and also because they look nice. The site sells both original and compatible tubes for modern and old devices, so if you are a bit of a flea-market hunter, you’ll be happy to know that for less than $10 you’ll probably be able to have that old radio up and about working again.
TubeDepot.com is very nicely designed (a true rarity in the electronic replacement parts segment) and is easy to use; I was happy to see that as part of the checkout process, users can choose to have their tubes tested for performance (not just to check they actually work, but also how well they do), which I reckon is a simple and great way to build trust and confidence from their shoppers. One of the things I liked the best about this site is that is carries some useful information on technical aspects of audio quality which seems definitely very convenient when trying to decide what tube to buy. 
