Marvel Unlimited lets you read a huge number of Marvel comics online for a mnthly or annual subscription. Doesn't cover 100% of their back catalogue, and you'll wait a while for current issues to appear, but ideal if you're not a hoarder of comics, or just getting started in reading old issues etc.
Usual price is $10 per month, or $60 per year; but there's a promotion going on with the Thor movie that gets you 30% off the annual price for the first year only. Code is THOR302011.
--Also worth noting, Marvel Unlimited is NOT compatible with the iPhone or iPad, either through Safari, or the Marvel apps. Which is a major pain, but doesn't stop this being a good deal!