Well hows about this ol’ lady. Big Mama Thornton, a huge influence in blues and jazz.https://www.youtube.com/watch?v=wxoGvBQtjpM
Taj Mahal
He has some nice stuff, but from what I’ve heard I wouldn’t say he sounds like Muddy Waters.
One of his songs got a lot of play on Beaker Street, but I’ve never been able to find the name.
Ruby Wilson, the “Queen of Beale Street”.
I’ll throw in another recommendation for a great Chicago Blues artist which (surprisingly) hasn’t been mentioned before: Buddy Guy, who was heavily influenced by Muddy and played in his band and on many of his recording sessions.
No mention of Charley Patton? Or Tommy Johnson (other than in “O Brother Where Art Thou?”)?
Patton and TJ are the REAL Kings of the Delta Blues. Check out www.bluesimages.com for superior prewar 78 reissues & annual calendars, t-shirts, etc.
I never heard of them. I shall check them out.
Robert Johnson is King of the Blues, darn your eyes! 
Re: Taj Mahal
Never said he sounds like Muddy Waters. OP is looking for others he might like. And if you like Muddy, you’ll like Taj Mahal. Here he is doing some Delta Blues. Statesboro Blues.
Do you recall any of the lyrics from the song you heard on Beaker Street? I might know it.
I think most people were focusing primarily on suggesting urban blues artists given that the OP was interested in Muddy Waters. But rural blues can be just as compelling or more so.
Unfortunately I do not.
My own take on Muddy Waters is that the stuff on Real Folk Blues and More Real Folk Blues was him at his most compelling. It has that mid-centry urban blues sound, sure, but is not really that divorced from the country blues Muddy Waters grew up with. But if you can accept that as a baseline, then I think the following blues performances cluster around there:
Howling Wolf
Jimmy Reed
John Lee Hooker (his early stuff particularly - his later stuff is good but I would cluster differently)
Elmore James
Bukka White
There is other excellent stuff that gets further from the Muddy Waters Real Folk Blues baseline. Hard to say where to make the cut off. Some things I didn’t include:
Lightnin’ Hopkins - I didn’t include him because mostly I have the sense of him as a singer-story-teller with an acoustic guitar, though I believe he was on the same circuit with other such guys. The linked song is closer to the baseline mentioned than a lot of his stuff.
Blind Willie Johnson - Older blues, and in particular any blues where the white noise of an older recording technology stands out, I tend to group differently. The linked to song is the one that was placed on the Voyager probe, as an example to other worlds of humanity’s musical output.
Blind Lemon Jefferson - Speaking of old. And country.
Big Bill Broonzy - Country blues.
What I know about blues came from buying those $4 tapes you used to get at Walmart and other places. I would get other stuff by musicians I liked and other versions of songs I liked. I also spent time at our university library listening to their jazz collections and bringing my own tape machine to make my own copies. Well, now you don’t have to maintain a collection of cassettes. If you like a song, check YouTube to see who else has covered it and how you feel about what they did with it. If you like an artist, see who else’s work pops up while you’re checking him out. It used to add up to a chunk of my paycheck to find this stuff out. Now all it’ll cost you is time.
Thanks to all! Great info education.