There's a bunch of stuff that is pre-order or only came with the Collector's Edition:
- AT-12 Raider - Origin pre-order bonus
- M-55 Argus - Pre-order bonus for select retailers
- N7 Warfare Gear - Pre-order bonus for select retailers
- Robotic Dog - Collector's edition
- Squad Outfit pack - Collector's edition
- N7 Weapons pack - Collector's edition
- N7 Hoodie - Collector's edition
TheThese are all single player items. The Collector's edition and pre-order bonuses are currently unavailable outside of pre-ordering the game or buying the collector's edition. No news on whether or not those of us with the "regular edition" will be able to buy them at a later date.
There's a couple of single player items that you can get for free by playing the Kingdoms of Amalur demo:
- Chakram Launcher
- Reckoner Knight Armor
There's a Battlefield 3 Pack (a set of multiplayer items) that you can only get by having Battlefield 3 activated to the same account as Mass Effect 3.
There's also the "From Dust" DLC (a single player mission + a new squadmate) which came with the deluxe/collectors editions but is available to anyone (from day 1) for around $10.
In addition to these items, there's a number of Razer ME3-branded items that come with special codes/unlocks for multiplayer.
On top of these, there's a whole set of Mass Effect 3 toys that you can buy, and each one comes with a similar code for multiplayer to the Razer items.
One tally of all the DLC for Mass Effect 3 listed the grand total at $870! However, the codes that come with the Razer gear and the toys are more along the lines of the "booster packs" you can buy in the multiplayer in-game shop, so I wouldn't consider them anywhere approaching essential.