I love my Ass Saver
An Ass Saver is a piece of plastic you can strap to your bike as an easily-removable temporary fender. They sell em with funky patterns and stuff.
I vastly prefer these removable fenders to proper installed ones. Right now it is raining like hell here in LA... but normally we are dry. More typical fenders usually attach at 3 or so points on the bike, each, so screwing them on and taking them off for just a couple days of rain a year is a bit of a chore.
The other thing I like about it is that the one I got is retroreflective! Most of my biking takes place at night or in the evening, so having anything shiny is very helpful.
And finally: it folds up! It folds into thirds, which is small enough to get it inside the sling bag that I use for biking. I have a small purse-sized bag with a "compression strap" I can use to bind it tightly to my back, and the ass saver fits so neatly inside that thing that it feels like a joke. It should be impossible for me to carry a damn fender around in my bag, but I can! Pretty sick!
I actually own two Ass Savers. One is much smaller, and clips under the seat of my fixed gear bike. It's small enough that it is no big deal for it to stay on all the time.
This is probably the best dollars-to-utility thing I have bought for my bike. If you live in a city where it rains all the time it probably sounds ridiculous to listen to me say "Wow! You should have a fender!" but here in LA it's really not that obvious that you should have a fender. When it's rainy here, a lot of people just swap to a different commute mode, or just suffer through it and get soaked and nasty. I work from home nearly all the time so I just choose to stay inside when it's raining. When it rains only a couple times a year, putting the extra weight of a fender on your bike is just not required.
And having this foldy bit of plastic has made it even less required for me!