Clickbank has some Wordpress plug ins as well: http://www.clickbank.com/help/affiliate-help/affiliate-tools/clickbank-wordpress-plugins/ although I don't know if you can use them with other products.
wp ecommerce is ok if you only want something simple... (you can make your own internal themes also, to get around paying for it.. eg grid views etc you have to go 'gold'.. but if you modify it to your own its a-ok as far as i know) it does the job i've needed it for anyway...