LÖVE

I discovered this today, it seems pretty useful! You can use it to generate tile sheets if you're on OS X. There's a free version that limits the amount of sprites you can use, but the full version only costs £1.19 from the (ugh) Mac App Store. Aparrently it works well with Box2D, but I'm not sure how that applies here.

http://spritehelper.wordpress.com/

There's also a level editor by the same guy, but I doubt it's very easy to use with Löve out-of-the-box.

If you're just interested in making a texture atlas/spritesheet from a folder of sprites then this seems to do the trick quite well, plus it's cross-platform, although it has much less features than that.