Adobe Kuler

This is Adobe's free online colour schemer.  It
has some great features such as generating your
colours by examining a picture that you like
(from Flickr or from your computer).  You can
also work out your colours manually.

The site has tons of users already and if you
don't want to work out your colours on your
own then you can look through the thousands
of already created schemes.

Sound Cloud

A little bit like YouTube for audio.  Musicians (or
producers, DJs, rappers, whatever) can sign up
(for free or paid premium) and upload their
music.  This is a great way to find new artists
and some of the work on here is absolutely
excellent.

If you like Dubstep I recommend you look up
Inofaith.

My music can also be found on this site.  My
account is located here.

Aviary

Aviary is an online image suite.  Basically it is
Adobe Creative Suite in your browser, for
free.  Keep in mind that that's me saying
that and not them.  They could probably be
sued for saying that.

This is great for those times when you are at a
friend's house and need to do some image
editing, only to find out he has nothing of use
on his (or her) computer.  Enter Aviary.

Either that or you are just bored :)

Loop Labs

A great little site for making music when you
don't have access to a proper DAW.  The
web apps on this site are loop sequencers
and they work fairly well.

I've made a couple quick songs on this site
when I needed to amuse myself.