There are many ISPs and there is no implicit reason why you cannot use almost every one on a Mac, However some ISPs state they do not support Mac OS X so check before you buy! If you are confident setting up your Mac and only require the actual configuration settings then a Mac-friendly ISP may be of less importance to you. However it is worth bearing in mind that if you need to ring the technical support you will often get very little assistance unless they have a Mac support team.