After beating the game, you can head over to the Dreamyard in the southern part of Unova to find either Latios (Black 2) or Latias (White 2), who will want you to chase them all over the Dreamyard in order to catch them.
The full instructions for this are covered in the Dreamyard section of this walkthrough.
After chasing them down, you'll have a chance to battle them!
After beating the game, you can return to Victory Road, where the Zoroark once blocked your path, and follow the path to N's Castle. You'll run into N there, who will follow you along through the ruins of the castle until you reach his old throne room.
You'll need to fight N, who uses either Reshiram (White 2) or Zekrom (Black 2). His Pokemon is at level 70 and can be rather challenging to take down.
Once you've beaten him, he'll give you the Light Stone (White 2) or the Dark Stone (Black 2), which you then need to take to the top of Dragonspiral Tower, where you'll be able to use it to awaken Reshiram (White 2) or Zekrom (Black 2).
Reshiram Lv. 70 White 2 only.
Zekrom Lv. 70 Black 2 only.
After you've caught them, you can return to the Giant Chasm to find the legendary Pokemon, Kyurem, waiting for you there.
Once you've caught (or knocked out) Reshiram or Zekrom (depending on your version) after beating the game, you can return to the back of the Giant Chasm where you fought against Black or White Kyurem and Ghetsis.
Kyurem Lv. 70
Capturing or defeating Kyurem will cause it to leave behind the DNA Splicers — an item that can be used to fuse Kyurem with either Reshiram or Zekrom.
Assuming you've gotten the Lunar Wing from the Strange House outside of Reversal Mountain, after beating the game, you can visit the Marvelous Bridge just to the east of Nimbasa City. You can actually visit the bridge without the wing, but if you have the Lunar Wing in your inventory, you'll encounter Cresselia on the bridge, ready for you to catch it!
After beating the game, you'll be able to access Clay Tunnel from Driftveil City, which is a rather large cave with plenty of Trainers to battle and items to pick up.
At the very back of the tunnel, you'll find what appears to be an empty room with a curious circular pattern by a wall. From the center of that circular pattern, you have to head six steps south, then nine steps to the east. Face south (without taking a step south) and press A to find a hidden switch. Trigger that and you'll open up the door to Regirock's chamber.
Regirock Lv. 65
After catching or knocking out Regirock, you'll be rewarded with a special Key depending on your version. You'll receive the Iron Key in Black 2 or the Iceberg Key in White 2. This Key can be shared with anyone using the Unova Link feature, but you only get the one Key on your version alone.
While you have the Iron Key active, you'll be able to head to the back of the room and have a chance to catch Registeel.
Registeel Lv. 65 Requires Iron Key
Additionally, while you have the Iceberg Key active from your Unova Link, you can encounter Regice instead.
Regice Lv. 65 Requires Iceberg Key
In order to get all three of the Regis, you'll need to use the Unova Link feature to exchange Keys with the version opposite the one you're playing, or just trade for the other Regi you're missing.
After you've caught all three of them, you can take them to Twist Mountain to trigger an event where you have the chance to catch Regigigas.
After picking up the Magma Stone on Route 18, which is only accessible after beating the game and surfing far west of Route 1, you can return to Reversal Mountain. There's a basement area that Bianca showed an interest in during your first visit there. Well, if you have the Magma Stone, you'll run into the legendary Pokemon Heatran there!
Running into Azelf, Mesprit, and Uxie requires a bit of exploration. It also requires you to have beaten the game.
After beating the game, head on over to Route 20 with a Pokemon that can use both Surf and Waterfall. From the Virbank City side of Route 20, surf across the water on the southern part of the route and then head west until you find a waterfall. Climb it, then follow the stream until you find a cave — the Cave of Being.
Inside there, you'll trigger a scene with Professor Juniper and the legendary trio of the lakes, Azelf, Mesprit, and Uxie, resulting in them running off. After you've done this, you'll be able to find the three of them in various locations across Unova, although hiding.
Azelf can be found by returning to Route 23, entering from where the path to Victory Road is. From there, head west, past a shrub that can be cut down, until you find some boulders by some steps leading up to a little platform. Take a few steps around on the platform there and Azelf will reveal itself, allowing you to fight it!
Azelf Lv. 65
Mesprit is a little easier to track down, although may take more time. You have to return to the Celestial Tower and climb up to the very top of it, where the bell is. Up there, you'll have to step on a certain spot somewhere in the southwest corner of the top of the tower in order for Mesprit to appear.
Mesprit Lv. 65
Uxie is the last of the three that can be found, requiring you to travel all the way through southern Unova until you reach Nacrene City. Take a few steps around the outside of the museum, notably to the southwest of the entrance, and you should find the spot that triggers Uxie to appear, ready for you to fight it.