[mjgoins]

If you like Sleep's Dopesmoker, the next logical step is Conference of the Birds, by OM.

[goldbrick]

Electric Wizard's Dopethrone is another venerable classic.

[WaxProlix]

Dopethrone is definitely the way to go if you like Sleep, and Electric Wizard's Witchcult Today offers some slight stylistic branching out while retaining the doom metal vibe, if folks are interested in further listening.

[ionised]

OM are also excellent.

Same vocalist and bassist as Sleep (though I think the bassist is different now and they've added a drummer).

[poloniculmov]

Al Cisneros is still the vocalist/basist/leader. The original drummer, who also played in Sleep left the band in 2008.

[thenipper]

I feel like the Body is also good place to go.