These links allow clients to manage invoices, review site maps, and securely exchange files.
I use freshbooks to track time and manage invoices. Clients can login to review and pay invoices.
I use jumpchart to quickly prototype new website layouts. Clients can login to review and comment on site layouts.
Jumpchart
I use box.net for storing and exchanging files. Clients receive their own secure drop-boxes for each project.