Hey, I've been looking for a forum like this, really appreciate everyone

To kick things off from my part:
Bad Romance - Dirty Big Lead (Logic)
Bad Romance - 100 ORG Another Drawbar (Sylenth)
Bad Romance - 117 DR Woodblock (Sylenth)
Telephone - Pretty much all orchestral instruments like Orchestral Harp and staccato violins (Logic)
Born This Way - The edgy synth lead is from Spectrasonics Omnisphere - Aggressive Digital Buzz 1
Someone mentioned this briefly, but Umbrella - Rihanna has Vintage Funk Kit 03 from GarageBand however it is layered with an 808 clap and kick.
Love in this Club - Usher also has two Garageband loops, Euro Hero Synth 02 and 03.
Let the Bass Kick - the titular male vocal is from Norman Cook - Skip to my Loops (Sample CD)
Bounce - Timbaland Tempo has reached critical level sound effect is from Zero-G Datafile 2 (Sample CD)
Price Tag - Jessie J has a loop sampled from Zimba Ku - Black Beat, however it needs some editing. That loop is perfectly edited in Spectrasonics Stylus RMX, under Classic Stylus, 75 Plate a, leading to my suspicion that Stylus was actually used. Pretty neat
I can also send snippets of these!
Some of these finds might've been mentioned elsewhere, but it's always good to have them compiled here.