I was wondering if there are any good recommendations on apps that takes screenshots of the iPhone simulator. I've tried apps like iPhone screenshot cropper but I'm looking for something that will allow me to make larger images. For example large enough to put on a poster. Any suggestions?
Unfortunately you're still going to be limited to the native resolution of the iPhone (320x480), no matter what you use. To use something like this for print, which is generally around 300dpi, your options aren't great, i.e. resizing in Photoshop...
You take the screenshots at the iPhone's resolution, and then you use interpolation to upscale your images. Many programs allow you to do that, but I agree with n3wtz that Photoshop is one of the best. I personally used it with remarkably good results.
In terms of actually capturing the image itself from the simulator, you could do worse than Grab, which comes with OS X.
Why not just use Command-shift-4? Taking Screenshots in Mac OS X
No matter what you grab it with you still only get the resolution of the 320 x 480 pixels.