Try this first. Great place to sample bands and some of my favorites are on there.
Ummm…
For the more rough, rock sound, I would check out And You Will Know Us By the Trail of Dead, The Party of Helicoptors, Need New Body, and Bent Leg Fatima.
Now… for Cursive… just look into Saddle Creek Records. The Faint, Despercaidos, Now its Overhead… all good by my book.
For a more smooth, mellow rock, just check out Elephant 6. Some great bands are found on there like Apples in Stereo, Of Montreal, Olivia Tremor Control, Neutral Milk Hotel… yea you get the idea. For the same sound on a different label try Badley Drawn Boy, I am Kloot, Daryll Ann, and maybe Gorky’s Zygotic Mynci.
For something like M83, umm… check out Physics or even GYBE!. I usually do not get into instrumental but those are two that come to mind.
Now, album recomendations. I think you should look into The Unicorns - Who Will Cut Our Hair When We Are Gone (you can find MP3s of their record label, Alien 8, website), Olivia Tremor Control’s Dusk at Cubist Castle, and my new favorite: Liars - They Were Wrong So We Drowned (you can listen off their website www.liarsliarsliars.com)
The Unicorns - Has a real fuzzy warm pop sound mixed with morbid lyrics. Very experimental.
OTC - Very experimental, smooth and has sounds from the '60s.
Liars - Hard to say if you will like them. They used to be a really good NYC “dance” indie band but they were tired of their fanbase so they released that album recently to make everyone mad. Its all about witches and folklore and has some genuinely good moments.
Hope this helps…