Some proprietary protocol from the camera manufacturer
Of the three USB Mass Storage is preferred, because it means that you can plug the camera in, and any reasonably modern operating system (any GNU/Linux system running a Linux 2.4 or later kernel, WinXP, or - I think - Mac OS X) will just detect it and you can access it as if it were any other removable media, like a floppy disk, CD, or USB flash drive. In fact it's the exact same protocol used by USB flash drives and similar devices.
Support for PTP can be had by installing gPhoto2, the "back end" for supporting PTP and a range of more obscure camera protocols. You will also want to install a program like gtkam, a user-friendly graphical interface for gPhoto2.
If you have a camera which uses a protocol other than USB Mass Storage or PTP, and which isn't supported by gPhoto2, you've got a very expensive paperweight. It's not even a good idea for Windows users to buy hardware which depends on proprietary protocols. There's no end of hardware that was released in the Win9x days which is unsupported on XP because Microsoft isn't interested, the hardware manufacturer has gone out of business, or would prefer you bought the latest model than kept using your old hardware.
Any camera you buy these days has sufficiently high resolution to produce good quality A3 prints, and camera manufacturers have stopped competing on megapixel numbers. So it's reasonable to expect that you'll use a camera you buy today until it stops working. How many operating system upgrades will you have gone through in that time? Will anyone see any profit in producing proprietary drivers for the camera by then?
I always tell people to ask their camera retailer whether the camera you're looking at uses the USB Mass Storage protocol. The salesperson won't know, of course, but it is such fun knowing precisely when you're going to get a blank look from someone. In fact the camera manufacturer probably won't even include that information in the camera's manual.
This quite comprehensive list of cameras is probably the best source of information you're going to find, and indeed it tells us that the Canon PowerShot A610 is supported by gPhoto, using either PTP or Canon's own protocol.
Camera Protocols
There are three ways a digital camera might talk to your computer:
Of the three USB Mass Storage is preferred, because it means that you can plug the camera in, and any reasonably modern operating system (any GNU/Linux system running a Linux 2.4 or later kernel, WinXP, or - I think - Mac OS X) will just detect it and you can access it as if it were any other removable media, like a floppy disk, CD, or USB flash drive. In fact it's the exact same protocol used by USB flash drives and similar devices.
Support for PTP can be had by installing gPhoto2, the "back end" for supporting PTP and a range of more obscure camera protocols. You will also want to install a program like gtkam, a user-friendly graphical interface for gPhoto2.
If you have a camera which uses a protocol other than USB Mass Storage or PTP, and which isn't supported by gPhoto2, you've got a very expensive paperweight. It's not even a good idea for Windows users to buy hardware which depends on proprietary protocols. There's no end of hardware that was released in the Win9x days which is unsupported on XP because Microsoft isn't interested, the hardware manufacturer has gone out of business, or would prefer you bought the latest model than kept using your old hardware.
Any camera you buy these days has sufficiently high resolution to produce good quality A3 prints, and camera manufacturers have stopped competing on megapixel numbers. So it's reasonable to expect that you'll use a camera you buy today until it stops working. How many operating system upgrades will you have gone through in that time? Will anyone see any profit in producing proprietary drivers for the camera by then?
I always tell people to ask their camera retailer whether the camera you're looking at uses the USB Mass Storage protocol. The salesperson won't know, of course, but it is such fun knowing precisely when you're going to get a blank look from someone. In fact the camera manufacturer probably won't even include that information in the camera's manual.
This quite comprehensive list of cameras is probably the best source of information you're going to find, and indeed it tells us that the Canon PowerShot A610 is supported by gPhoto, using either PTP or Canon's own protocol.