There are quite a few Excel blogs but these are the ones I really keep an eye on!

Mike Woodhouse:  Grumpy Old Programmer

Dennis Wallentin: VSTO & .Net & Excel

Jon Peltier: PTS Blog

Doug Jenkins: Newton Excel Bach

Dick Kusliker: Daily Dose of Excel

JimmyPeña: Code for Outlook and Excel

Mike Alexander: Bacon Bits

Simon Murphy: Smurf On Spreadsheets

Bob Phillips: Excel Do, Dynamic Does

Rob Collie: PowerPivotPro

Chandoo:  Pointy Haired Dilbert

Debra Dalgleish: Contextures Blog

Tom Gleeson: Goban Sao