Khan Academy: Extremely popular right now. Videos and explanations for many subjects, searchable by topics and even by problems in some cases. Very easy to follow, understand, and search for your topic.
Idroo.com: I've never actually used this, but it looks great--virtual white board, free, and usable via Skype. Online tutoring??
Google chat and google docs: increasingly easiest way to collaborate online and communicate quickly. Majority of students have google accounts already, and it's easy/free to sign up if not.
Open Educational Resources: I've only just started browsing here. Index of OE resources, both print and digital. Expanding.
myatp.org: Association for Tutoring Professional journal for news, info, and updates.
NCDE: National Center for Developmental Ed. You can browse the reports and research section for free...sometimes not as updated.
CRLA: College Reading and Learning Association. Tutor certification program, including training and resources (with membership). Useful links section is the only really useful part if you're not a member/applying to be a member.