Akismet is the default anti-spam plugin which comes with WordPress and it has saved me from literally hundreds of thousnads of comment spam messages (124,200 last time I looked).
A new version (Akismet 2.0) was released the same time as WordPress 2.1′s release so it’s release was kind of drowned out in the hoopla.
To my mind, the most significant change in Akismet 2.0 is the ability to tell Akismet to automatically delete any comments on posts over a month old.
As Matt himself said:
When I was doing some research into false positives I found an interesting statistic: the overwhelming majority (more that 99.99%) of false positives (which is when Akismet marks someone as spam wrongly) occur on new posts. Which makes sense because most real comments happen on new entries.
Typically I used to get >500 comments per day flagged by Akismet. There was no way i could go through those looking for genuine comments accidentally flagged as spam by Akismet.
Today though, having configured Akismet to dump all suspected spam comments on posts over a month old, I now only have to check 20-30 comments per day.
Well done to the guys in Automattic again. I love Akismet.