I remember viewing a page having monthly achievers, daily top rankers or something like this. But now I am not able to find a link to that. May be its just in front of me and I am just overlooking it.
We don't have a list of monthly achievers or anything. However, we do have a tag stats page, which lists the top scores in participation in the tags for both question askers and answerers. There is a list for the scores over the course of the last 30 days, and one for overall. You can access these when browsing any tag by clicking the "stats" tab at the top.
An example, for C# on Stack Overflow: https://stackoverflow.com/questions/tagged?tagnames=c%23&sort=stats&pagesize=50
Check out this answer for a look into how score is calculated.
Originally, tag stats would allow combinations of tags like [java]+[swing]. This was apparently never intended and caused problems, so the functionality has been removed.
Reputation leagues are now rolled out:
Two bits that Grace's answer didn't mention:
- The tag stats without specifying any tags: https://stackoverflow.com/questions/tagged?tagnames=&sort=stats&pagesize=50
- codeka's monthly league Stack App: http://www.codeka.com.au/Solt/League/SO/2010/7