National Institute of Educational Technologies and Teacher Training (Spain)
Ministerio De Educacion: El Sistema Decimal De Numeracion
Practice your knowledge of different number systems with the interactive example then check your knowledge with the different exercises.
Purple Math
Purplemath: Rounding Numbers
Reviews how to round numbers to a given place or number of decimal places.
Math Aids
Math Aids: Number Systems Worksheets
Great resource to practice and strengthen math skills pertaining to number systems. Math worksheets are printable, may be customized, and include answer keys.