YouTube has a "Watch Later" function, but there are some problems with it. First and most obviously, it only works on YouTube, which is not all the video on the web. For anything that's not on YouTube, you're out of luck.
It will preload to your Android phone, when you're on WiFi and charging, but still needs a network connection to watch. That's not "offline", it's buffering. It also makes me nervous about its actual mobile data usage. Despite preloading on WiFi, a recent train trip cost me 129MB of mobile data on the YouTube app, clearly identified by Android's bandwidth tracking. That's more than half my monthly quota. So it's an unreliable and unpredictable feature, too.
It can't be used to defer videos before they start playing. If you're on 3G, you will take a data hit as they start buffering, then you change your mind. It also seems that the app doesn't keep a list of what it's preloaded, so managing your list when your connection drops is not possible.
In general, it feels like they've taken a step towards fixing buffering issues and high data usage on slow networks, but that same step could be used for a truly great offline experience, and they have refused to go that far. Presumably "offline" video starts making official rights holders nervous about downloading and piracy. YouTube has a delicate balance to strike, between appearing to be a "safe" place to upload video that is streamable but not downloadable (which is a fallacy in itself - streaming is a form of downloading) but also treating their patrons as valuable, which is impossible if they are also the attackers in YouTube's security scenarios.
Mokalus of Borg
PS - I won't be using it again unless I've got WiFi on the train, which is pretty rare.
PPS - Or if I'm watching on my desktop, which makes the queue less useful.