ToodleDo: This is the best online task management system I've tried. It has a mobile app which syncs, supports tasks with complex recurrence patterns, and is pretty easy to use. I put all my life's tasks in here, even small ones, just to get them "out of mind."
Google Calendar, Docs, Gmail: These are sort of no-brainers. They sync nicely with my phone and allow me to share my data with my family and classmates.
Twitter, LinkedIn, Facebook: I check these once a day and use them just for networking/contact info and keeping up with my friends. I'm really not a power user of these yet.
Delicious: Nice site for bookmarking with tagging and private bookmark capabilities. I rarely remember that I have a Delicious account when I run across something I like, but when I do, I'm happy about it. It'd be nice if it could save a snapshot of the page in case it gets taken down, but I'm sure other services out there do that.
Flickr: I've been trying to get into photography more, and I host some of my work here. I generally just upload completely raw/unedited files that I'm proud of. I have a good friend who is an inspirational photographer who puts up TONS of photos on Flickr; maybe one day I'll be as cool as him. The site is easy to use and their desktop uploading app is pretty handy. I haven't played much with the social features of Flickr.
SoundCloud: Only used this once for my own mix, but it was a breeze to set up, and browsing the site had a really nice UX.
Issu: This is a site that lets you "publish" your own high-quality, full-screen readable documents/magazines (a bit like Scribd). I've used it a couple times and enjoyed it.
DropBox: I've started using this to collaborate on files a lot since starting at UCLA. It's much more easy to use than Google Docs for files that live on my computer and across computers, like Excel and Word files that I don't want to convert to Google Docs yet.
Kindle, Audible, LibraryThing, Netflix: These are my "entertainment" sources. I've had the pleasure of reading my Kindle books across a number of devices and enjoy the page syncing across them. I've recently been trying out Audible and have been impressed by how many books I could potentially "read" in my car (especially on double speed). I only wish that one "purchase" of an Amazon book allowed me to read it (in sync) across Kindle and Audible together. I use LibraryThing to catalog the books I own and need to read and the books I want to purchase/borrow and read in the future. Netflix is what I use for movies on DVD and streaming (though I love going to the theater too). I wish on-demand films came out day-in-date with the theatrical release and feel that will eventually happen. I don't really watch TV or play games anymore, hence the lack of those on my entertainment roster.
If there are sites or services you think I'd find useful or interesting, definitely leave a comment.