You've got steemvoter, streemian and steemauto.

Personally, I use steemauto. It has all the features the rest have and it is a free service.
The only reason steemvoter was ahead initially was because you could set rules to number of upvotes a day but steemauto just added that recently so i guess they're both equals now.

Streemian is really cool too, has a free service and a premium service but still, steemauto still does everything it does so I'll rather stick with steemauto.