Noteorious: Note reading game and has options for all clefs.
Music Theory Review: There quite a lot on this website, some may be beyond what you have done so far.
MusicTheory.net : If you explore the three pages listed along the top on the left side for “Lessons, Exercises, and Tools” there are many things to explore on this website for free.
8notes.com:This website has a lot of free music for you to explore. Can be good for sight reading!
Violin Masterclass: The Sassmanhaus Tradition of Violin Playing More geared for violin/viola players, but still lots of good information and videos!
The Music Interactive This site has several good (and free!) apps to download to PC/iOS (Staff Wars). Look through the "Classroom Apps" and "Performance Apps" and see what might be of interest to you! (This has STAFF WARS!)
Cello Bello Cello geared, but very good information about technique, musicianship, artistry, etc!
String Skills Lots of wonderful information about all the string instruments, sightreading, shifting, technique, you name it... it's probably here..
Musical Instrument Museum Virtual Tour: Interesting website that lets you take a virtal tour of the museum.
Google Music Lab: There are many "labs" to explore. The "Song Maker" lab is particularly fun to play around with.
Practice Tips: This website has some really wonderful practice tips that can help you get the most out of your practice time.