In order to ensure that SE always has someone capable of acting as a functioning admin and maintainer of the codebase, the idea of subscriptions and other small payments were introduced. This will allow SE's custodian (currently steele) to treat SE as a part-time job. It's an old idea, going as far back as matt and the very first iteration of the SE codebase.
As long as SE can afford to be here it will be free for the average user, but for those who help financially a couple of extra perks have been added into the mix.
Silver Level: Achieved via Upfront or Monthly Subscription, or Gifts
Special Mods: Currently +1 laz0r, +1 Funsightful, +1 Sad
The ability to turn off ads.
Early Access to supported frill features.
Collapsible Comment Threads.
A toolbar for HTML in comments.
More to come as v2 of the codebase gets underway in the coming months!
The ability to click directly to specific comments from the admin or search page.
Exemption from your karma score decaying on days you don't visit.
Gold Level: Achieved via Upfront or Monthly Subscription
All of the Silver Perks.
1 Custom Mod of your choice that you can use in a thread and make available to everyone else in that thread. You can be the gatekeeper to your own personal mod of awesome!
1 Personal banner ad that will be among the pool of other subscribers' and steele's ads.
Platinum Level: Achieved via Upfront or Monthly Subscription
All of the Silver and Gold Perks.
Monthly Silver Gifts to give out to other users. Gifts expire at the end of 30 days.
One Time Payments are Currently suspended until sometime this summer. Message steele if you absolutely need to do a one time payment, but recurring monthly payments are currently preferred. It's this whole "thing" with Student Loan Payments. :P