OK I posted about series I was in the middle of here in July 2007. This is the updated list with edits in blue. Some things have changed, many have not:
1) Wen Spencer - need to start book 4 of 4 in the Ukiah Oregon series
2) Wen Spencer – need to start book 2 of 2? maybe in the Tinker series – done!
3) Patricia Briggs – read 2 3 out of 7(!!), in the Mercy Thompson series
4) Kim Harrison – need to start book 3 of ??? in the Hollows series
5) Rachel Caine – need to start book 3 of ?? in the Weather Warden series
6) Rachel Caine - read 2 3 of ?? in the Morganville Vampires series
7) Dawn Cook – need to start book 2 of 4 in the Truth series
8) Kat Richardson – need to start book 2 3 of ?? (3 so far) in the Greywalker series
9) Faith Hunter - need to start book 2 3 of ?? (3 so far) in the Rogue Mage series
10) Michelle Sagara – read 2 3 of ?? (5 so far) in the Cast in series (book 4 will be out October 2008)
11) Garth Nix – started reading his Keys of the Kingdom series (book 1). Waiting for book 7 of 7 to go through it since its fast reading. YA.
12) Anne Bishop – I read book 1 of her Ephemera series (prob a trilogy), not really interested in continuing. Maybe.
13) Mindy Klasky - Need to read book 4 of 5 in her Glasswright series.
14) Moira J. Moore – Need to read book 2 3 of ?? (around 4 so far) in her Hero series
15) Anne Bishop – Read her Jewels trilogy, need to read her 2 other books from this world (continuations?) - Dreams made Flesh, Tangled Webs..
16) Sharon Shinn – Read book 1 of her Twelve Houses series.. not sure if I want to read 2 and 3 out of ??.
17) Karen Chance – Read book 2 of ?? in her Shadows series.
18) S. L. Viehl - need to start book 1 4 of 8 so far in her Stardoc series
19) Marie Brennan – Reading book 2 of 2 in her Doppleganger duology (I think) done!
20) Louise Rennison – Read book 2 of ?? (7 so far) in her Georgia Nicolson series
21) Casey Daniels – Read book 2 of ?? (at least 3 so far).. in her Pepper Martin series. Probably will not continue. gave up!
22) Ilona Andrews – Read book 1 2 of 2 ??? so far of her Magic series
23) Lilith Saintcrow – Read book 2 3 of 5 in her Dante Valentine series
24) Karin Lowachee – Read book 1 of 3 in her series, will continue eventually.
25) Katie MacAlister – Read up to book 3 of her Aisling Grey, Guardian series, not really interested in continuing. gave up!
26) Janet Evanovich - Read book 1 of … ??? 13 so far in her Stephanie Plum series. Not interested in continuing. gave up!
27) MaryJanice Davidson – Read book 1 of ?? in her Undead series
28) Holly Black – Read book 3 of 3 in her Modern Faery Tale series. done!
29) Kristen Britain – Read book 2 of 3 (I think) in her Green Rider series.. Not really into it.. may finish though.
30) Ellen Schreiner – Read book 2 of ?? of her Vampire Kisses series gave up!
31) Sophie Kinsella – Read book 2 of ?? (5 so far) of her Shopaholic series
32) Kelley Armstrong – Read book 1 of ?? (8 so far) of her Otherwold series
33) Stephenie Meyer – Read book 1 of ?? (3 so far) of her series
34) Clare B. Dunkle – Read book 1 of 3 in her Hollow Kingdom series
New:
35) Patricia Briggs – Read book 1 of ??? in her Anna and Charles series
36) Nalini Singh - need to start 4 of ?? (5 so far) in her Psy Changling series
37) Shanna Swendson - need to start book 3 of 4 in her Katie Chandler series
38) Jasper Fforde – Read book 1 of ?? in his Thursday Next series
39) Jeri Smith Ready – need to finish book 2 of 3 (I think) in her Aspect of Crow series
40) Jeri Smith Ready – Read book 1 of ?? in her vampire DJs series
41) Patricia Briggs – Read book 2 of 2 in her Hurog series – done!
42) Maria V. Snyder – Read book 3 of 3 in her Study series – done!
43) Liz Berry – Read 2 of 2 in Connections duology – done!
44) Ann Agguire – Read 2 of 4 (so far) of her Jax series
45) Jocelyn Drake – Read 1 of ?? in her Nightwalker series
46) Jennifer Estep – Read 3 of ?? in her Bigtime series
47) Ally Carter – Read 2 of ?? (3 so far) in her Gallager Girl series
48) C. E. Murphy – Read 1 of ?? (3?) in her Negotiator series
49) C. E. Murphy – Read 1 of ?? (3?) in her Walker Papers series
50) Kelley Armstrong – Read 1 of ?? in her Nadia Staffor series
51) Tate Hallaway – Read 2 of ?? (3 so far) in her Garnet Lacey series
Looking at this list: Wow, I feel so tired!! I really need to finish off some series before I can start another. I have the rest of certain series just lying in my TBR mountain. Ahhhhhhhhh. I wish more series weren't just "open-ended" without a set number of books expected…
Ahhhhhhhhhhhhhhhhhhh!!
specficromantic 
I did something similar around the same time I believe. I have caught up with a couple of the series, and I am waiting for new books in some of them, but I have added more than I have finished!
Yes, it creeps up on you too. I swear I have the best intentions and then I look and I realize: ok I'm in the middle of a LOT of series, I really have to finish them!