Soft amount definitions:
i.e. A payment up to and including "n" includes the user in the appropriate group.
Automatic badge assignment, possibly creation:
For sites which are donation funded (i.e. a donation doesn't unlock some feature or content), generate and assign a badge to each user who donates, with the badge reflecting the level of donation. On the one hand it gives public recognition of the users who have donated, on the other it gives new users an idea of how much people are donating.
An Admin interface showing basic stats about donations:
Total count of donators (with a link to lists, complete, and by level of donation).
Total/Average amount of Donation(s).
Recent Donations (today, this week, this month, this quarter, this year).
Upcoming renewals (this month, this quarter).
A User page showing the current totals and averages, with an optional manually configurable "It costs X$/year to run this site"
An optional (soft) nag screen on TL promotions "Hi, welcome to TL_3, 98% of TL_3 users have donated money, with an average donation of 23.50$. Since you obviously enjoy using www.allaboutaxolotls.com, why not pony up? We'll even award you a really cool badge!"