Nagios v2.1 Stable has just been released and can be downloaded from here. The following are the changes from v2.0:
Changed freshness logic so that passive checks don’t immediately go stale after program restart
Bug fix for minor memory leak in object cleanup code
Bug fix for flapping notifications during scheduled downtime
Bug fix for $TOTALHOSTSDOWNUNHANDLED$ macro
Bug fix in sample minimal.cfg file
Bug fix in status CGI when displaying servicegroups
Bug fixes in computation of indeterminate time and scheduled downtime in availability CGI
Bug fix with not deleting all comments associated with a service
Lowered max plugin output length from 348 to 332 chars to run on 64-bit systems without problems
Minor fix to p1.pl for embedded Perl interpreter
Minor fixes to WAP interface (statuswml CGI)
Minor bug fix to VRML interface (statuswrl CGI)
Minor doc updates