This looks interesting. The Printed Page is hosting a "Perpetual Challenge" for readers of series books. The web site is called To Be Continued, and the challenge has no time limit (that's the perpetual part). So far, these are the guidelines (but it's still evolving, so check the site for further info):
No books lists or sign ups; cross over books and challenges are welcome, actually encouraged
If not based on the main character central characters must play a prominent part throughout the series
All genres welcome – YA, historical, western, paranormal, mystery, police procedural, adventure, etc.
No trilogies please; only ongoing series (see Spotlight Series)
Many of the books I read are parts of a series, so I'll definitely be getting in on this one.