Nicer clothes. This one depends a lot on how style-conscious you and your usual environs are, of course, and probably a lot of people here don't need to be told this. On the other hand, I'm sure there are also people here like I was a few years ago, when I not only didn't care about style or fashion, I also basically just wore whatever was most cheap and comfortable in any given scenario. That was a mistake.
I found that having some respectably-nice-but-not-too-formal clothes can be a big plus in an environment where everybody expects a t-shirt with jeans/shorts. Think collared button-down shirt and slacks, with some nicer-than-everyday shoes. You don't need many sets of such clothes - the idea is to have something nice to wear when you expect it to be noticed, not necessarily to overhaul your whole wardrobe - but for things like a party that a friend is throwing where you know there will be people who you haven't met, it's worth the investment. This is especially true if you're looking for a new relationship.
It may take some work, if you're not used to it, to figure out the right balance of formality and style. That's OK. In my social circles it is often very easy to be the best-dressed guy in the room. A suit is too much. A coat and tie is too much. A $30 shirt and a $40 pair of slacks, plus some nice-ish shoes, will go a long way.
Dark wash jeans can often be more versatile than slacks for situations that aren't businessy.
What can I purchase with $100 that will be the best thing I can buy to make my life better?
I've decided to budget some regular money to improving my life each month. I'd like to start with low hanging fruit for obvious reasons - but when I sat down to think of improvements, I found myself thinking of the same old things I'd already been planning to do anyway... and I'd like out of that rut.
Constraints/more info:
Background:
This is a question I recently posed to my local Less Wrong group and we came up with a few good ideas, so I thought I'd share the discussion with the wider community and see what we can come up with. I'll add the list we came up with later on in the comments...
It'd be great to have a repository of low-hanging fruit for things that can be solved with (relatively affordable) amounts of money. I'd personally like to go through the list - look at candidates that sound like they'd be really useful to me and then make a prioritised list of what to work on first.