Ambleside Primary has done it again! In addition to their hundreds chart and their multiplication chart, they also have an interactive number line. The number line comes with 7 colored squares that you can use to cover the numbers so that students have to calculate what number is missing. This would be great on a Promethean board, too! Notice at the bottom of the site, there are blue buttons with options for different types of number lines: 1's, 10's, 100's, 0.1's, 0.5's, -5 through 5, or create your own under the section entitled "Make a line". Just specify the number you want the line to begin at and the increments to count up by and click "Go". You end up with your own personalized line! If it the site allowed you to display more than 11 numbers at a time, it would be even better.
The Math Worksheet Site has a number line generator, but instead of it being a digital, interactive image, it creates a worksheet that you can print out. The site does not limit the values you can put into the number line, so some number lines may print out very small. This is a nice option for teachers to know about who use number lines to teach basic math. If you have a Promethean board, you can generate the number line, and then use "Annotate Desktop" to write over the number line at the board.