Once you finish TimmyK's site, this is the one I've been using most recently for other Picross puzzles. :O
www.griddlers.net/
It includes multicolored puzzles, multi-grid images, "triddlers" (triangle-based instead of square based), and grids of any size from 5x5 up to 50x50, both square and rectangular.
When compared to TimmyK's site:
Pros
You can save puzzles.
Wider variety of puzzles
It gives you an indicator when a row matches the blocks. (Not that it's necessarily right, but that you've accounted for every block given in that line.)
Cons
You can't cross off numbers.
The interface is more difficult to get a handle on.
Also, for a while I thought you couldn't mark areas at a time, but I discovered just recently you actually can by holding down Alt when you click. You can also shift-right-clicking to leave a circle (shift-left-click to remove it), but this doesn't keep you from marking the square as filled. This way you can use it as a marker for various purposes.
Filling in squares works like Paint in this version; you have a left-click and a right-click color. There is a bar on the right side that displays every color used in the puzzle. Left click one to pick up that color for left-click filling, and the same with right-clicking. The first color is always "erase" (neither empty nor filled), and the second color is always the "empty" square color.
Also, an important note for if you do a multi-colored Picross puzzle here: You do
not need an empty square between two lines of squares of two different colors. Only if they are the same color. <p><hr />
"Never let your morals get in the way of doing what is right" - Salvor Hardin
</p>