What I do: I add anything associated with a date to Google Calendar.

For to-do's on my main project I use Google Tasks.

For ongoing, less important to-dos or ideas I keep a draft email and just add to the list. I reopen it and keep it minimized when I'm working from that list.

For short-tem stuff that I want to knock out I keep it in my inbox so I can knock them out quickly when I have time or I'm motivated to get a few things done.

Urgent/quick stuff I try to take care of the first time I look at it.

I send myself a quick email for everything so I don't forget, then sort it into the appropriate place above when I get a chance.

I generally stay ~close to inbox zero. This system works for me because I like everything in once place and accessible from anywhere. Additionally, all of my 5+ email accounts feed into my Gmail inbox. I don't use Gmail tabs or anything like that, I just spam/unsubscribe from most things.