Rainforest Math (see "place value," "ordering," "rounding"

Place value
Place value e-lab
Place value demo 
Bead Numbers   (to hundreds only)
Place Value (About)
Place Value Playoff
Place Value Pirates
MathCats Really Big Numbers
Decimal and whole number jeopardy
Place value, math.com (About)
Write big numbers
Interactive Math's place value
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
Numbernut (see rounding & estimating section)
Place Value Puzzler
Rounding demo
Rounding flashcards
Skillswise rounding and estimating
Glowla's Estimation Contraction
BasketMath Rounding

Ordering and comparing
Comparing numbers using a number line
The Number Line
Ordering numbers e-lab
One False Move (order numbers)
Comparing through millions
Skillswise Greater than, Less than or Equal
Skillswise Order Numbers
Ordering 7-digit numbers
Comparing numbers (various levels)
Awards Ceremony
Number lines

Negative numbers
Negative number ordering
Line Jumper (number line)
CyberChase (negative numbers)
Reading a thermometer
Color chip addition of negative and positive numbers
Number lines (select -5 to 5)