I have taken posession of a Samsung Galaxy S 2.

Problem:

In early June my battery life plummetted from being able to fully charge the phone over night and then recharge again at night to only lasting 4hours or less.

Applications:

I had the following apps installed

Astro File Manager Pro with Bluetooth & SMB
Cloud Print
Google+
Instagram
Tiny FlashLight
Box
iSilo

It's Not Just Me:

My boss has a Samsung Galaxy S and he is seeing the same symptoms.

My Solution:

I did two things

1. Remove and Replace the Battery (I don't think this fixed it but I'm putting this here to be complete)
2. Reset the phone to defaults.

Resetting phone to defaults:

Menu => Settings => Back up and reset => Factory data reset

I chose to "Erase USB Storage" aswell

Results:

Today at one oclock  my battery is at 65% (viewed from Menu => Settings => Battery)

Suspicions:

I believe an update to some Google Software such as the recent update of "Market" to "Google Play" may be causing the issue but I've been unable to get definitive proof. If anyone finds out exactly what the problem is please let us know via a comment.

Going Forward:

So now I'm going to try and re-install each app one at a time to see if it was indeed my applications causing the problem.