Lightnin’ Slim, Lazy Lester, Jimmy Reed
Not sure if Alligator records is still around. Their first album was Hound Dog Taylor and the Houserockers. Awesome if you like your blues raw and electric. The Dog had 6 fingers on the fretboard (look closely at the albumn cover).
Lot of Alligators stuff and compilations are real accessible to a beginner that wants more.
Here’s everything you need to know about how to sing the blues.
I played with this guy several times:
Probably one of the most fabulous blues harmonica players ever!
Highly recommended.
Jas. Mathus and His Knock-Down Society
Most famous for his work in the (defunct?) Squirrel Nut Zippers, he has since put out two albums under this new moniker: Songs for Rosetta and National Antiseptic.
The thing I like about what Mathus is doing is that he is reviving the raw grit and earthiness of pre-rock blues and makes it palatable for a contemporary (blues) audience.
I prefer this direction over the modern Stevie Ray Vaughan electric guitar style that is most prevalent in blues today.
I have to put in a word for Keb’ Mo’. See him live if you can. If not, pick up Just Like You, The Door or his self-titled debut album. Well, you’ll pick them up after the live show, anyway. 
Other contemporary blues artists I like are:
Tab Benoit
Gov’t Mule
R.L. Burnside
Popa Chubby
Scott Holt
I highly recommend “Great Moments with B.B. King” as an intro to his stuff.
Also, If you want Blues “light” or a more mainstream approach, seek out anything by Howard And The White Boys.
It’s kind of upbeat and very fun to listen to.
If you’re into Blues-Rock like Hendrix and Zeppelin, you’ll definately want to spend some quality time with Eric Clapton. Cream was meant to be primarily a blues band with rock influences. More recently, his album “From The Cradle” is pure blues and is the biggest selling blues album of all time.
I’m also a big fan of Stevie Ray Vaughan and Double Trouble. Try “Texas Flood” or “In Step.” Also, Double Trouble just put out an album of their own last year with some fine music on it.
For some other interesting flavours, try some Etta James, lots of Buddy Guy, and take a listen to some older Bonnie Raitt. For my money she’s one of the best slide guitarists around.
Oh, and for a Canadian take on the blues, try Colin James’ “Sudden Stop” or The Jeff Healy Band’s “See The Light.”