What does Mad Moxxi's tip jar in Sanctuary do? You can tip her either $100 or $1000.
An achievement and guns. (No cutscenes.)
The "Sugar Daddy" achievement unlocks at $10,000 cumulatively donated. You can exit the game without saving to keep the achievement and your money.
Moxxi hands over two guns (and variants whereof):
- Miss Moxxi's Bad Touch, a red, corrosive SMG
- Miss Moxxi's Good Touch, a purple, incendiary SMG, vibrates your controller when in use
The Bad Touch triggers between 5k and 20k, and the Good Touch between 10k and 50k (when tipping in $1000 increments). This amount could be random—reports are inconclusive.
You can return to (or reload) Sanctuary for more Good Touches, but reportedly, there is only one Bad Touch per playthrough. The variants (e.g., "Guileless" and "Impetuous") tend to have lower stats. The guns' level reqs scale to your story progress. You can always restart your game from the last save if you don't like your results.
According to this link if you give her 15 000$ in tips you could get a vibrating gun that make the 2nd controller vibrate all the time for the ladies :D
Note: you could get 2 weapons: Moxxi's Good Touch (as described above) or Moxxi's Bad Touch, which doesn't vibrate.