I’ve been using Gmail’s mutliple inboxes to manage my email for about a year now - and it’s the only kind of task management that has ever worked for me. You can enable it in Settings > Labs, and configure it in Settings > Multiple Inboxes.

Labels

In a nutshell, multiple inboxes display Gmail search results beside your general inbox. Gmail supports search queries like “label:sideprojectA” - and that’s what makes multiple inboxes so great.

I’ve used some variation of the following method for about a year, but even after trying to reduce the complexity, I’ve always come back to my current setup. I’ve split up my inboxes into the follow 4 labels, stacked on top of my general inbox.

• Requires Reply - This email needs a reply right away.
• Requires Attention - This email doesn’t need a reply, but you need to take some action (like a GoDaddy domain renewal notice)
• Requires Checkup - Make sure to email this person and ask how things are going.
• Thumbtack - The equivalent of adding a “Star” to messages, but for less important/more temporary things. I use the star for important messages I’ll want to come back to in the future. 

I’ve used some variation of this for a while, and at one point I tried reducing the complexity to just a “Requires Reply” label. Eventually, though, I ended up back at this format. As a person who’s conflictingly disorganized and obsessive, this is the only system that’s lasted.