Go get it here. Hopefully I'm not the first one to point this out to you. But better late than never. This service pack addresses issues that have come up with Visual Studio 2005 since it was first released just over a year ago. You can read more about VS 2005 SP1 here.
A while back, I posted about Visual Studio 2005 support on Windows Vista. If you're on Vista... there's still another update coming for VS 2005 in addition to SP1. That will be the separate "VS 2005 SP1 Update for Windows Vista". The final release of that update is due out sometime in Q1 2007, but you can try out the beta here.
[Update:] Thanks to David for pointing out in the comments that this is a big install. You might want to let this one run when you leave the office for the day. Either that or head over to Dunkin' Donuts for a huge mug of coffee!