Cosmigo Promotion has a Tilemap feature. It's very handy and basically what it does it autocomplete tiles as you work on them:
Imagine you have a level made from tiles drawn out and you wanna change a tree somewhere, now if you change those tiles all the other instances get changed as well.
You can also specify x and y flipping and a limit with up to 9999 tiles, plus it can export a few formats of tilemaps and makes a tilesheet for you.
I know I am sounding like an advertisment here, but I am sure other people can attest to what I said, and I have used this professionally since quite a while.
I can attest to this. I got into game development recently, and thought to try out the tilemap features before even getting a map editor: it works beautifully. In tilemap mode theres a "sync mode" checkbox, when its checked editing one tile edits all of them, when its unchecked, you can draw freely, and check it again to generate any new tiles that might have been made. Ontop of this, its an extremely great pixel editor, and exports to a variety of formats. For example, the TXM formats integrate seamlessly with flixel, but any programmer worth his salt would be able to write a script to convert them to whatever is needed.
Making things even more convenient is the brush container: if you have a few smaller tiles you usually use in conjunction, you can just add them as a brush and draw at will, very useful.
Here is a screenshot i took while making the cave ground tiles, although I feel I should suggest against doing 8x8 tiles: it gives a lot of flexibility, but sure is tedious when you're making hundreds of screens. Also note that the thumbnail/preview window can be zoomed too! I have it at 2x since thats how my graphics are displayed.
The only problem might be simplicity, it can take awhile to really learn your way around the program if you're not used to it. Its not exactly MS Paint, its very powerful, but requires some learning to use.