I play a Gibson Firebird V, and right now I'm playing a Line 6 Spider III 15w through a Digitech RP90 Multi-Effects Pedal. What would be a good amp for me to play with? I play bluesy stuff, along with delta blues, some folk, and a bit of rock 'n roll/metal.
What's a nice, powerful amp that would fit my needs?
For the styles you have listed, a Fender Blues Jr. would be ideal. It handles pedals well, and its built on distortion is a nice bluesy punch.
Main Setup:
Charvel So-Cal

Dunlop Volume
Digitech Tuner
Custom Compressor
VFE Blues King
Ibanez TS9 30th Anniv
Boss DD20
Boss NS2
MXR 10 Band EQ
Hardwire Supernatural
Loop-Master 2 Channel Looper

Mesa Single Rec
Any amp that makes sound.

Bigger speakers work better, generally, though. Your current amp has an 8 inch speaker- go with something with a 10, 12 or 15.
Alex quoted it before I could, so what he said.

Specifically though, as an owner of one, I can tell you here that from Texas Blues to Memphis Blues to Chicago Blues to British Blues and everything in-between, if you have a slab of wood with an output jack on it, the Blues Junior will make it sing unless you hate US-voiced amps.

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Fender Blues Jr
Peavey Classic 30
Peavey Delta Blues-I'm a bigger fan of this over the other two(I own one).
Really depends how much money you want to spend.

On the cheaper side, you can go for the Fender Blues Jr or the Peavey Classic as many people have said.

Of the two I'd go for the Blue Jr.

On the downside. The Blues Jr starts to sound like crap at louder volumes, and the 3 Peavey Classics I've gone through all had reliability issues, and they ate through tubes like they were candies.

If you want to spend a little more money then you should check out:
Fender Deluxe
Fender Twin
Fender Blues Deluxe
Hughes and Kettner Statesman (It's like a Peavey Classic, except far far better)
Mesa Express

For even more money

Mesa Lonestar or Lonestar Special
Fender 57 Twin, Tweed, or or Deluxe
Soho 65
Dr Z Maz 18
Matchless (never tried one, don't know much about them other than a lot of big blues players use them)