Place value
Place value e-lab
Place value demo 
Number systems
Place Value (About)
Place Value Playoff
Place Value Pirates
MathCats Really Big Numbers
Decimal and whole number jeopardy
Place value, math.com (About)
Place Value Puzzler
Cookie Dough
Place value information and practice
Expanded form to standard form
Place value to standard form
Identify the place value
Identify all the place values
Number Machine
Place Value Game

Rounding and estimating
Rounding Off
Place Value Puzzler (Choose "Round Number")
Rounding flashcards
Half-court rounding
Rounding master
Glowla's Estimation Contraction
BasketMath Rounding
Numbernut (see rounding & estimating section)

Ordering and comparing
Comparing numbers using a number line
The Number Line
Ordering numbers e-lab
One False Move (order numbers)
Comparing through millions
Ordering 7-digit numbers
Awards Ceremony
Number lines

Negative numbers
Line Jumper (number line)
CyberChase (negative numbers)
Number lines (select -5 to 5)
Integer Tilt
Integer Tilt 2