Overview of ARM-MCUs
Good overviews can be found at the Wikipedia lists for ARM processor cores and Applications of ARM cores.  You'll probably want to filter your search significantly - Trying to find a basic 32-bit controller in the ARM7TDMI or Cortex-M3 series is a very different task than trying to find a multicore applications processor for your consumer product.

Of course, if you're looking for certain specifications, distributor parametric searches can be helpful.  For example, both Digikey and Mouser have Microcontroller (1, 2) and Microprocessor (1, 2) pages with good selection tools.

Finally, some compiler/IDE/software providers maintain lists of the processors they support.  For instance, Keil's device database has information about these ARM based chips.

Those links notwithstanding, if you want to exhaustively compare manufacturers, you will have to make some phone calls.