There are a number of candidates, including interim councilmember Anita Bonds (she was appointed to hold Mendelson's seat temporarily,) former Ward 1 candidate and current school board member Patrick Mara (who was also a co-owner of Meridian Pint), former City Paper "Loose Lips" writer Elissa Silverman, ANC commissioner Matthew Frumin, marijuana lawyer Paul Zukerberg, and activist Perry Redd. (Two other candidates withdrew or were kicked off the ballot.)
That's a lot of people -- to help, DCist, PoP and Greater Greater Washington have set up Let's Choose DC, a site that poses questions to the various challengers. Silverman seems to be getting the highest ratings on that site.
Mara, however, seems to have the early lead, at least in terms of endorsements: he's been endorsed by the Washington Post, Sierra Club, and DC Chamber of Commerce. The Post recently described him as the front-runner, and that he's "fiscally conservative, socially liberal, pro-education," but that he also has a lot of critics and apparently makes a living by selling political trinkets on eBay. Kind of wacky.
In the endorsement, however, it said he brings a break from the past, as opposed to Bonds, the other assumed front-runner, who has a long career, including campaign manager for one of Marion Barry's mayoral campaigns.
So, do your own research at Let's Choose DC, and I'll write later about who I'd recommend you vote for.