Tough one because there are probably a few things that I've just forgotten about.
There was a brief period where I regretted giving away my PlayStation 1 but honestly, it was probably a good thing. I wouldn't have touched it and whoever got it probably enjoyed it a lot more than I would.
I'm sure there are lots of things I regretted throwing away/giving away for a time but it was probably better in the end. There are something things, however, that you think "Oh, I could have used that just now!" after throwing/giving it away.
Nothing really. There was stuff I had some feelings about, but if it was actually that important, it wouldn't have wound up in the discard pile. A week later, I couldn't have even told you what I was so worried about tossing, let alone still missed it.
I sort of regret giving away my Super Nintendo (SNES) which could have got me some money at gamestop. Which they are now taking old consoles for cash and trade-in. I don't suppose I regret it too much because the person I gave it to is a friend. But would of got me a few bucks.
Some wheels/bearings off a skateboard my dad got for me at a really difficult time - i still have the actual board itself and the trucks, but it'd still be nice to have the whole thing and it feels like I disrespected my dad a bit to have done that (but I was 12 when I gave them away so I didn't really think of it like that).