Here’s a script you can use with GreaseMonkey or Stylish that removes those pesky Gmail sponsored link adverts on the right hand side.
If I had a pay-and-remove option then that would be something I’d seriously consider however this is all Gmail Help has to say on the matter:
Opt Out of Ads
Gmail users can’t opt out of receiving ads because these sponsored links help Google support the cost of providing Gmail for free to our users. Instead of serving pop-ups and untargeted banner ads, Gmail displays text ads using our contextual advertising technology. These ads should be relevant to the content of your messages and we hope you’ll find them useful.
Note, an update since I originally posted this – Google have released Google Apps Premier Edition which lets you opt out of ads and also gives you a whopping 25GB of space. All for $50 / user account / year.