Some of the most important factors when choosing a payment processor include your business location (i.e. are you based in the USA, India, China etc.) as this could affect the choice of your payment processor as well as fees and volume; some processors will charge the same amount for each transaction and some will give you a discount based on the volume of your sales.
Some of the most popular are:
http://www.paypal.com http://www.moneybookers.com http://checkout.google.com http://www.worldpay.com http://www.2checkout.com
There are also some local processors which deal with clients from specific areas, like NoChex in the UK for example.
Reality suggests that the best options are either PayPal, Moneybookers or Google Checkout.
