Here's a good site for practicing rhythms.  You can either listen to a rhythm and then notate it,

or you can look at a written rhythm and listen to several examples, trying to match the written

rhythm to what you are hearing.

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eMusicTheory

Here's a site that allows you to practice performing rhythms, notating rhythms, naming notes

on the staff, identifying intervals, identifying key signatures, and various other musical concepts.

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Big Ears

This site is an online ear trainer.  It plays an interval, and your job is to figure out which interval

it played.  You can limit it to a few types of intervals, or you can select all intervals (I recommend

starting easy!).  Check it out, it will drastically improve your sight-singing!

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MusicTheory

Here's another site for music theory.  This site has tons of lessons on various topics, as well as

training tools for concepts such as intervals, note reading, key signatures, and even guitar and

keyboards.  If a trainer is too hard or too easy, make sure you check the settings and change

as necessary.

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Vocalist

This site has TONS of information for singers!  Scroll down to see the list of options available. 

There are various exercises, information on diction and foreign languages, technique pointers,

and lots of other things.  Take some time to check out all this site has to offer.