I heard that you can get a Geth assault rifle somewhere? Im bored sick of using the Vindicator and in general bored of the small weapon count in this game.

By hidden weapons, Im wondering about the ones that only show up on certain difficulties. I guess this could be the hidden weapons thread if there's more than one hidden weapon.

Geth Assault Rifle is located near the end of Tali's Recruitment mission. The only way to get it, however, is to start and play through the mission on Hardcore difficulty or greater. 
 
There is also the Revenant Light Machine Gun, an upgrade only available to soldiers--you acquire it on the deactivated Collector Vessel where you would normally, with any other class, pick up another weapon specialty.
 
I'm pretty sure there's a Geth Sniper Rifle available at some point. Not sure if that's a secret weapon or if that's just a Legion-specific upgrade.

Infiltrators get a bad-ass sniper rifle on that Collector vessel, too.  I think It's the same type of rifle Legion gets.  Don't know if it's Geth though - it says in the description that it's not designed to be compatible with a human's shoulder.

Revenant Machine Gun (from "disabled" collector ship) > anything else in the entire game
 
Well except maybe the Cain.....because it looks some god-damn COOL when it goes off:

Geth sniper is just Legion's anti-materiel rifle. For Shepard, you have to pick it on the collector ship if you're soldier or infiltrator.

EDIT: Guess Garrus beat me to that bit.

@Garrus:

Geth rifle is the best assault rifle, but it has a fluctuating fire rate, which is weird. The revanent machine gun is by far the best gun in the class, but if you skipped it on the ship, you'll never get it. low accuracy, but high damage and 80 round sinks, plus it gets tons of ammo off every pickup. Otherwise, no. No secrets. Just promised downloads. There are 2 weapons in each class by default, plus the Geth assault rifle, the three special weapons, and now the cerberus shotgun. Oh, and the collector rifle for CE, and the incisor for digital deluxe purchasers.

On the disabled collector ship choice of:
 
extra weapon type slot  

or

Claymore shotgun for Vanguard & Soldier
Widow Sniper rifle for Infiltrator & Soldier
Revanant Assault rifle for Soldier only
 
 
Other weapons: 
Flame Thrower heavy weapon - Zaeed loyalty mission DLC
Geth Assault rifle - Tali recruitment on Hardcore and Insanity only (read above posts for specifics)
Collector Assault rifle - Collector's edition bonus
Incisor Sniper rifle - Digital Deluxe Edition bonus (Steam or similar digital distribution)
Eviscerator Shotgun - DLC via Cerberus network ?? (haven't checked for this since I finished the game)

You can also pick up Grunt's super shotgun up if you have it and then you get to the collector part where you can choose another gun.
 
That thing is powerful.

God, I love the Widow Sniper Rifle. That thing fires with feeling.

" Geth Assault Rifle is located near the end of Tali's Recruitment mission. The only way to get it, however, is to start and play through the mission on Hardcore difficulty or greater.   "

Oh My God, I need to play this again don't I...!!!

" @Shiro_Shishi: Yeah you can equip Grunt's shotty, I never actually tried it yet but will do soon! "

If you already played the game once and got the revenant, Can you get another weapon on the collector ship? Or does it replace the one you have, or is there nothing there?

If you go into the collector ship as an Infiltrator, you have the option to get the same Sniper that Legion uses. It's called the Widow, and it's probably the best gun in the game. It's slow but so, so strong.

" If you already played the game once and got the revenant, Can you get another weapon on the collector ship? Or does it replace the one you have, or is there nothing there? "

" @Garrus: Really? It seems like that shotgun is for Grunt, even the description talks about only Krogan being able to use it. "

I honestly like the shotguns from Mass Effect a lot more. It's not that they are superior, but I had my shotgun equipped with a long barrel that made the accuracy go like thirty feet, plus incendiary rounds.

" @Napalm: I think the whole thermal clip thing has ruined the shotguns in Mass Effect 2. Just sounds kinda silly when you have a class that's main weapon is a shotgun and your best shotgun probably, has like 12 shots or something when you're facing 30 mechs. "