FUCK I ACCIDENTALLY HIT CMD-R INSTEAD OF CMD-T AND RELOADED AND LOST ALL OF MY TEXT AGHHHHHHHH
Google Calendar is the best!!! It's how I keep track of all of my appointments. I always have a browser window open with my calendar in addition to syncing with my phone. 20 minutes before an event, I get a notification to my phone and also a pop-up on my laptop. So it's pretty hard to ignore! And, if you add a location, Google will automatically calculate your transportation time to there from where you are using your preferred transportation method and will notify you 20 minutes before you should leave! It's amazing.
Plus, you can create separate calendars and choose which ones to view at any given time. So I have a calendar set up for all of my meetings, but then I have a second calendar where I block out chunks of time to do different tasks. That way, I can easily toggle between viewing everything I need to do for the week, and various subsets of that stuff. Plus, you can color code!! For example, this is what my calendar looks like right now:
Pink represents classes, green represents research-related meetings, light purple represents social activities (of course, usually social things are planned last minute so usually they don't make it into the calendar), orange represents talks, and dark purple represents chunks of time I've allotted to do particular tasks. I keep some time open because of course unforeseen tasks come up during the week!
So I use it both as a calendar and as a task manager because I just add tasks I need to do as dark purple color-coded events. However, I also sometimes make use of to-do items and reminders, also through the Google Calendar interface. You can add to-do items and reminders that appear in the top portion of the calendar for a particular day (instead of having an assigned time to them). And it you use Inbox (the new Gmail interface, right now it's invite-only), you can have reminders emailed to you as well, or "snooze" them so they're sent at a later specified time.
Then the next step is to convince all of your friends to use it and share their calendars with you so you can easily figure out when everyone is available to hang out instead of going through so much hassle to plan things...
Edit: Also, I highly recommend keeping a spreadsheet where you track how much time you spend in various categories of work. That way you can figure out where you're spending too much time, or not enough, and then compensate. I just have a simple spreadsheet with various categories like each of my classes, each of my research projects, etc., and a couple times a day will update how much time I spent on each one that day. Then once you've been doing this for a few weeks you get a good feel of where you're spending your time and you can adjust accordingly. I've found that this really, really helps my productivity!!