Grand Ayatollah Yousef Saanei has effectively succeeded Grand Ayatollah Hossein Ali Montazeri (God grant him peace) as the spiritual guide of the opposition. Here’s more from The Century Foundation’s insideIRAN.org:

Now, stepping into the vacuum left by Montazeri’s death is another prominent reformist ayatollah who has emerged to provide the Green Movement with spiritual guidance and ideological support: Grand Ayatollah Yousuf Saanei declared on December 20 his desire to continue Ayatollah Montazeri’s work and honor his legacy, assuming his mantle as the most prominent clerical reformist. His statement may be found here .
Ayatollah Saanei is known for his dynamic involvement in contemporary issues that serve as sources of contention both politically and theologically. He supports complete legal and social equality for women and condemns both terrorism and nuclear proliferation as un-Islamic.
Saanei’s credentials in both the theological and political spheres are considerable. He is recognized as a grand ayatollah and a source of emulation for Shiite Muslims. His revolutionary background is also notable, having been a trusted supporter of Ayatollah Khomeini, the founder of the Islamic Republic. Saanei served in the Guardian Council, the Assembly of Experts, and the Judiciary branch in the early 1980s.

And, of course, the conservatives have reacted violently, ransacking his house and office in Qom (not to mention his office in Shiraz) just after Grand Ayatollah Montazeri’s passing. The authorities banned him from attending the late ayatollah’s funeral, but he showed up anyway. Hard-liners Ayatollah Mohammad Yazdi and the Qom Lecturer’s Association have declared him ineligible to be a Marja-ye Taqlid, or grand ayatollah (which has “no precedent in Shiite history—ayatollahs may confirm the religious ranking of a scholarly peer, but never have had the religious authority de facto to excommunicate or demote him from the ranking” ).
This has not stopped Grand Ayatollah Saanei, however…

Saanei has hit the ground running in terms of advising the Green Movement and its leaders. Mir Hossein Moussavi and Mehdi Karroubi stayed in Saanei’s house before attending Ayatollah Montazeri’s funeral, no doubt discussing the current situation with him during their stay. Furthermore, Saanei has been in constant contact with Moussavi.
Even before Montazeri’s death, Saanei and Moussavi exchanged letters discussing the philosophy and spiritual basis of their resistance to the government. Saanei’s statements also have been posted on Moussavi’s Facebook page in the past few months.
The hardliners’ impotent and largely symbolic actions against him have merely accorded him more status with the Green Movement and have yet to hinder his actions or agenda in any discernable way. Moreover, Saanei’s activism may prove even more dynamic than Montazeri’s, as Montazeri was hampered both by his advanced age and the house arrest imposed on him by hardliners. Furthermore, Montazeri’s house arrest was a result of his fall from Ayatollah Khomeini’s grace. The current Supreme Leader, Ayatollah Khamenei, lacks the spiritual credentials to do the same to Saanei. In short, Saanei is almost impossible to marginalize.