I'll list the ones obtainable in BW:
Rotom - Heat, Wash, Fan, Mow, Frost forms. Go to Route 9, into the Shopping Mall 9, and go into a little room on the bottom right. There are some appliances there, and let Rotom play with them. The one you choose determines Rotom's form.
Darmanitan - Zen Mode (hidden ability; you need to give one of the statues a Rage Candy Bar outside Desert Resort). When its health is half/ below half, it'll change into Zen Mode. When its health is restored, it'll revert to its normal form.
Deerling/ Sawsbuck - changes forms due to seasons
These you can still get in BW, but they only change forms in B2W2.
Thundurus, Tornadus & Landorus -
The kami trio change forms by using the Reveal Glass, which is only obtainable by capturing the Landorus in the Pokémon Dream Radar and taking it to the Abundant Shrine in Pokémon Black and White 2.
Kyurem - White & Black, use DNA splicers to fuse with Reshiram/ Zekrom, but only in B2W2.
Keldeo - Resolute form. Teach it Secret Sword at the pledge grove, but only in B2W2.
Form Differences - Bulba