Now that multiple versions of the Second Life viewer are available (since message liberation was implemented, last June in 1.18.0), Linden Lab is choosing to support two production releases of the viewer at a time. In this case, 1.19.1 and 1.19.0.
Users who are still using any of the 1.18.x series viewers will have to update from today to either the current 1.19.1(4) release from the download page or get 1.19.0(5) [links after the fold for those that need them].
While our own experiences with 1.19.1 have been pretty solid (except for the damnable memory leaks, which it seems just about every third-party viewer has managed to fix), we understand that not everyone's experience has been as good with this version.
For your convenience, we have the download links for 1.19.0(5) here, if that is the version you choose to install. Definitely the thing we have been asked for most today.
You may also prefer Nicholaz Beresford's most execellent hand-tuned viewers, which are quite capable of running for hours or days at a time with minimal memory leakage and smooth performance. Pending anything silly from Linden Lab, Beresford's viewers should still continue to run, even though they are based on the 1.18 code-base at present
